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/command.lua

@@ -25,6 +25,9 @@ M.subs = {
   follow = function()
     require("heph.link").follow()
   end,
+  link = function()
+    require("heph.link").insert()
+  end,
   open = function(args)
     if args[1] then
       require("heph.node").open(args[1])