feat(nvim): inline #hashtags become tags on save (§8.3)

On save, whitespace-prefixed `#hashtags` in a node's body are unioned
into its tag set (via `frontmatter.hashtags` + the existing tag diff), so
you can tag a note by writing `#kitchen` inline. A markdown `# heading`
has a space after the `#`, so it never matches. e2e covers it.

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

heph.nvim/lua/heph/node.lua

@@ -51,7 +51,7 @@ function M.write(buf, _uri)
     -- block ⇒ the user removed it; treat the whole buffer as body, touch no
     -- metadata.) The backend also strips defensively, so `body` is what stores.
     local canonical = M._canonical[buf] or {}
-    frontmatter.apply(id, canonical, fm)
+    frontmatter.apply(id, canonical, fm, body)
     if fm.title and fm.title ~= canonical.title then
       params.title = fm.title
     end