feat: node.linkable — first-class link targets for the [[ picker (§8.4)

The picker listed every node, so each task showed up twice (itself + its
same-titled canonical-context doc) plus tag/log noise — and the new
preview made the duplicates look identical. New `Store::list_linkable_nodes`
/ `node.linkable` returns non-tombstoned nodes minus `tag`s and the docs
that are a task's canonical-context or log attachment (you link the task,
not its body). The Telescope picker now sources from it.

Tests: a socket test (5 nodes → 2 linkable: task + standalone doc).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/changelog.d/v1-prototype.feature.md

@@ -28,7 +28,7 @@ Begin the v1 prototype (Phase 1, tech-spec §11.1), built in TDD slices:
 - `heph-tui` task-list visuals (§8.1): each row now leads with an attention **flag** (`⚑`, colored red/orange/blue; blank for white) and a **project-colored bullet** — the bullet's color is derived stably from the project id (so it survives projects being added/removed), letting you scan a mixed list by project at a glance. The list also grows a **scrollbar** and keeps the selected task scrolled into view when there are more tasks than fit.
 - `heph-tui` sort toggle (§8.1): **`s`** flips the task list between two orders — **default** (attention → most-overdue → project → creation) and **by-project** (grouped under dimmed `──── Project ────` separators, then the same sub-order). The view's filter still applies first. (To free `s`, **skip** moved to **`S`**.)
 - `heph.nvim` task-view rows (§8): `:Heph next`/`:Heph list` rows now show a compact **do/late date chip** (and a recurrence `↻`), so you can see scheduling at a glance; `<CR>` still jumps to a task's context doc.
-- Wiki-links by node id (§8.4): node resolution is now **id-first** (`[[NODEID]]` resolves to its node ahead of any name match, so links can't be shadowed by a like-named node), and heph.nvim grows a **`[[` picker** — type `[[` (or `:Heph link`) to pick a node and insert a canonical `[[NODEID]]` link. With Telescope it's a **live fuzzy filter over all your nodes with a preview pane** (the node's body, or a task's context doc) — type to narrow, Enter to insert, `<C-x>` to create a doc named what you've typed; without Telescope it falls back to a search-then-select prompt. Following such a link (`<CR>`) jumps straight by id. Those id links are kept **readable**: on read a bare `[[NODEID]]` is expanded to `[[NODEID|Current Name]]` (so it follows renames, in both the nvim buffer and the TUI preview), and on save it collapses back to the canonical bare id — a custom `|label` you write is preserved as an override. In the editor the id is **concealed** — a link renders as just its name, styled like a hyperlink, with the raw `[[id|Name]]` revealed on the line your cursor is on. Legacy `[[Name]]` links keep working, and **`heph migrate-links`** rewrites them to the canonical id form in one pass when you're ready (idempotent).
+- Wiki-links by node id (§8.4): node resolution is now **id-first** (`[[NODEID]]` resolves to its node ahead of any name match, so links can't be shadowed by a like-named node), and heph.nvim grows a **`[[` picker** — type `[[` (or `:Heph link`) to pick a node and insert a canonical `[[NODEID]]` link. With Telescope it's a **live fuzzy filter over your linkable nodes with a preview pane** (the node's body, or a task's context doc) — the list shows first-class targets only (a task appears once; its internal context/log docs and tag nodes are hidden, via the new `node.linkable` query) — type to narrow, Enter to insert, `<C-x>` to create a doc named what you've typed; without Telescope it falls back to a search-then-select prompt. Following such a link (`<CR>`) jumps straight by id. Those id links are kept **readable**: on read a bare `[[NODEID]]` is expanded to `[[NODEID|Current Name]]` (so it follows renames, in both the nvim buffer and the TUI preview), and on save it collapses back to the canonical bare id — a custom `|label` you write is preserved as an override. In the editor the id is **concealed** — a link renders as just its name, styled like a hyperlink, with the raw `[[id|Name]]` revealed on the line your cursor is on. Legacy `[[Name]]` links keep working, and **`heph migrate-links`** rewrites them to the canonical id form in one pass when you're ready (idempotent).
 - Frontmatter editing in heph.nvim (§8.3): opening a node now shows an editable **YAML frontmatter** block on top of the body (`id`/`kind`/`title`/`tags`, and for a task or its context doc the task's `state`/`attention`/`do_date`/`late_on`/`recurrence`/`project`). On save, the plugin diffs the block and issues the right RPC per changed field — rename, set-attention, reschedule (dates as `YYYY-MM-DD`), move-to-project (by name), and tag add/remove — then saves the body; the store strips the block so it never persists. A mistyped `state` surfaces a validation error; a buffer with no block changes no metadata (so deleting the block can't wipe your tags). Inline **`#hashtags`** typed in the body are also added as tags on save (a `# heading` doesn't count) and are rendered in **italics** so they stand out. Link-follow and promotion are unaffected (they're content-relative, not line-absolute).
 - Frontmatter projection (§8.3): a node can now be fetched with an editable **YAML frontmatter** block prepended — `node.get {frontmatter: true}` renders `id`/`kind`/`title`/`tags`, and for a task (or its context doc) the owning task's `state`/`attention`/`do_date`/`late_on`/`recurrence`/`project` plus a `task:` ref. Dates are local `YYYY-MM-DD`. On write, the store **strips and ignores** any leading frontmatter (conservatively — a real `---` hrule in prose survives) before the CRDT diff, so frontmatter never persists and an unchanged read→write is a no-op; a naive editor can't corrupt metadata. This is the read/write groundwork for editing a node's metadata as frontmatter in heph.nvim (the diff-into-RPCs layer is next).
 - Tags (§4, §8.3): nodes can now be **tagged**. A tag is a `tag`-kind node whose id is deterministic in `(owner, name)`, so the same name is **one canonical tag** shared across everything it's applied to (and replicas converge — no duplicate tags). Tagging is an OR-set link, so adding/removing is idempotent and merge-safe. Surfaced as `tag.add`/`tag.remove`/`tag.list` RPCs and `heph tag add|rm|list` (list a node's tags, or every tag with no node). Tag names are trimmed; a canonical case/spelling normalization is deferred to the future zk import. This is the groundwork for the `tags:` line of the upcoming frontmatter edit surface.

docs/reference/tech-spec.md

@@ -334,7 +334,7 @@ Field rules: `id`/`kind` are **read-only** (display only); `title`, `attention`,
 > **Status: id-addressed links + the `[[` picker are built**; read-expansion/conceal display and the legacy migration are next. Historically bodies stored the human text `[[Title]]` and links materialized by resolving name→id at write time, which is ambiguous (a task and its canonical-context doc share a title — hence the resolution hack in §6/`links::resolve_id`). The fix: **the body stores the canonical node id**, and **no name-addressed link ever enters the DB**.
 
 - ✅ **Resolution is id-first:** `links::resolve_id` checks for an exact live node **id** before alias/title, so `[[NODEID]]` resolves to its node (and a like-named node can't shadow it). Legacy `[[Name]]` links still resolve by name until the migration runs; the canonical-context exclusion hack therefore stays for now (removed once name-resolution is retired).
-- ✅ **`heph.nvim` authoring:** typing `[[` (or `:Heph link`) opens a node picker and inserts `[[NODEID|Name]]` (labelled → readable + conceal-ready; collapses to bare on save). With **Telescope** it's a **live fuzzy filter over every node** (`node.list`) with a **preview pane** (the node's body, or — for a task — its canonical-context doc, fetched via RPC) — type to narrow, `<CR>` inserts, **`<C-x>` creates a doc named the current prompt** (a miss flows straight into "make it"; mirrors obsidian.nvim's `new` mapping). Without Telescope it falls back to a `search`-then-`vim.ui.select` prompt with a "+ Create" entry. Follow (`<CR>`) resolves the id directly.
+- ✅ **`heph.nvim` authoring:** typing `[[` (or `:Heph link`) opens a node picker and inserts `[[NODEID|Name]]` (labelled → readable + conceal-ready; collapses to bare on save). With **Telescope** it's a **live fuzzy filter over the linkable nodes** (`node.linkable` — non-tombstoned nodes minus `tag`s and the `doc`s that are a task's canonical-context/log attachment, so a task appears once, not twice) with a **preview pane** (the node's body, or — for a task — its canonical-context doc, fetched via RPC) — type to narrow, `<CR>` inserts, **`<C-x>` creates a doc named the current prompt** (a miss flows straight into "make it"; mirrors obsidian.nvim's `new` mapping). Without Telescope it falls back to a `search`-then-`vim.ui.select` prompt with a "+ Create" entry. Follow (`<CR>`) resolves the id directly.
 - **At rest (target):** `[[NODEID]]`, or `[[NODEID|custom text]]` when the author wrote explicit display text. The id before the `|` is the target.
 - ✅ **Projection (same philosophy as §8.3):** `heph-core::wikilink` (pure, injected id→title) — `node.get` **expands** a bare `[[NODEID]]` → `[[NODEID|Current Name]]` (every read, so the nvim buffer *and* the TUI preview are readable), and `update_node` **collapses** a `|text` equal to the target's current name back to bare before the CRDT diff (a custom label is preserved as an override). Transform order: read = expand links → prepend frontmatter; write = strip frontmatter → collapse links → store. An unchanged read→write round-trips to the canonical bare id. *(`heph export` still emits raw ids — a later polish.)*
 - ✅ **`heph.nvim` display:** `conceal.lua` hides the `[[id|` prefix and the `]]` suffix with conceal extmarks (refreshed on edit), leaving the label as a styled `HephLink` hyperlink; `conceallevel=2` + empty `concealcursor` reveal the raw link on the cursor's line so it stays editable.