feat: wiki-links by id — id-first resolution + heph.nvim [[ picker (§8.4)

Backend: `links::resolve_id` now checks for an exact live node id before
alias/title, so a canonical `[[NODEID]]` link resolves to its node and
can't be shadowed by a like-named node. Legacy `[[Name]]` links still
resolve by name (until the migration), so this is additive.

heph.nvim: `link.insert` (bound to insert-mode `[[` and `:Heph link`)
searches via the `search` RPC and inserts `[[NODEID]]`, with a "+ Create
new doc" entry; `<CR>` follow resolves the id directly. e2e covers
search→insert→materialize and the create path.

Remaining (§8.4): read-expansion/conceal display + the one-time
[[Title]]→[[NODEID]] migration (then retire name-resolution + the hack).

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

heph.nvim/lua/heph/link.lua

@@ -68,12 +68,48 @@ function M.follow()
   require("heph.node").open(node.id)
 end
 
---- Attach the buffer-local `<CR>` follow keymap and inline-`#hashtag`
+--- Pick a node (by full-text search) and insert a canonical `[[NODEID]]` link at
+--- the cursor — the authoring path for wiki-links-by-id (§8.4); a node id is the
+--- only thing that ever enters a stored link, so there's no name ambiguity. A
+--- "Create" entry mints a new doc named after the query. No-op if cancelled.
+function M.insert()
+  vim.ui.input({ prompt = "Link to: " }, function(query)
+    if not query or query == "" then
+      return
+    end
+    local items = {}
+    for _, hit in ipairs(rpc.call("search", { query = query }) or {}) do
+      items[#items + 1] = hit
+    end
+    items[#items + 1] = { __create = true, title = query }
+    require("heph.picker").select(items, {
+      prompt = "Link to",
+      format = function(it)
+        if it.__create then
+          return "+ Create new doc: " .. it.title
+        end
+        return it.title .. "  [" .. (it.kind or "node") .. "]"
+      end,
+    }, function(choice)
+      if not choice then
+        return
+      end
+      local id = choice.__create and rpc.call("node.create", { kind = "doc", title = choice.title }).id or choice.id
+      vim.api.nvim_put({ "[[" .. id .. "]]" }, "c", true, true)
+    end)
+  end)
+end
+
+--- Attach the buffer-local follow/insert keymaps and inline-`#hashtag`
 --- highlighting (only on heph:// buffers).
 function M.attach(buf)
   vim.keymap.set("n", "<CR>", function()
     M.follow()
   end, { buffer = buf, desc = "heph: follow [[link]]" })
+  -- Typing `[[` opens the node picker (Obsidian-style), inserting `[[NODEID]]`.
+  vim.keymap.set("i", "[[", function()
+    M.insert()
+  end, { buffer = buf, desc = "heph: insert [[link]]" })
 
   -- Render inline #hashtags in italics so they stand out — matching the
   -- save-time tag detection (whitespace-prefixed `#word`, never a `# heading`).