heph.nvim: follow-or-create wiki links + :Heph doc

Pressing <CR> on a [[wiki-link]] whose target doesn't exist now creates a doc
with that title and opens it (the zettelkasten gesture), and materializes the
source's backlink: if the source has unsaved edits, saving re-extracts and
links it (and persists the edits); otherwise the wiki link is added directly
(a no-op re-save wouldn't re-extract). Adds :Heph doc <title> to create a
standalone wiki entry. e2e covers both the saved-source and just-typed-source
paths.

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

docs/reference/heph-nvim.md

@@ -50,14 +50,18 @@ Beyond the existing methods (tech-spec §6), the plugin relies on
 **`node.resolve {title} → Node | null`**: an exact, owner-scoped,
 non-tombstoned alias-then-title match — the same mapping the store uses to
 materialize `wiki` links, so "follow link under cursor" jumps to the *same*
-node the stored link points at.
+node the stored link points at. When the target doesn't resolve, follow
+**creates** a `doc` with that title (the zettelkasten follow-or-create gesture)
+and materializes the source's backlink (saving the source if it has unsaved
+edits, else adding the `wiki` link directly).
 
 ## Commands (as of slice 11c)
 
 | Command | Action |
 |---|---|
 | `:Heph today` / `:Heph journal <YYYY-MM-DD>` | Open today's / a dated journal |
-| `:Heph follow` (also `<CR>` in a node buffer) | Follow the `[[link]]` under the cursor |
+| `:Heph follow` (also `<CR>` in a node buffer) | Follow the `[[link]]` under the cursor — **creating** the target doc if it doesn't exist yet |
+| `:Heph doc <title>` | Create (and open) a new wiki doc |
 | `:Heph open <id>` | Open a node buffer by id |
 | `:Heph search <query>` | Full-text search; pick a result to open |
 | `:Heph next [scope]` | Tactical "what is next?" view (`<CR>` opens a task's context) |