feat(nvim): conceal wiki-link ids to styled name hyperlinks (§8.4)

`conceal.lua` hides the `[[id|` prefix and `]]` suffix with conceal
extmarks (refreshed on edit), leaving the label as a styled `HephLink`;
`conceallevel=2` + empty `concealcursor` reveal the raw `[[id|Name]]` on
the cursor's line so it stays editable. The `[[` picker now inserts the
labelled `[[id|Name]]` form (readable + conceal-ready; collapses to bare
on save). e2e asserts the conceal extmarks + conceallevel.

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

heph.nvim/lua/heph/node.lua

@@ -34,6 +34,7 @@ function M.read(buf, uri)
   vim.bo[buf].fileformat = "unix"
   vim.bo[buf].modified = false
   require("heph.link").attach(buf)
+  require("heph.conceal").attach(buf)
 end
 
 --- `BufWriteCmd` handler: route frontmatter edits to RPCs, then save the body.