fix(html-embed): address code-review findings on the sandbox commit

Follow-up fixes to the htmlEmbed-sandbox / trackerHead change:

- share-seo: inject trackerHead via a function replacer so `$`-sequences
  ($&, $', $`, $$) in the admin snippet are inserted literally instead of
  being treated as String.replace substitution patterns; warn when the
  </head> marker is absent instead of silently skipping injection.
- mcp: register a passthrough `htmlEmbed` node in the schema mirror so an
  AI/MCP edit of a page containing an embed no longer throws
  "Unknown node type: htmlEmbed" in TiptapTransformer.toYdoc.
- editor-ext + client: treat a non-finite `data-height` as auto (null) so a
  crafted/corrupted height cannot disable auto-resize or yield a NaN iframe
  height; extract a shared clampHeight helper.
- client: rename render-raw-html.{ts,test.ts} -> html-embed-sandbox.{...} and
  shouldExecute -> shouldRender so the seam name matches the sandbox model.
- client: i18n the iframe title; surface the real error reason in
  tracker-settings (console.error + err.response.data.message).
- docs: note hasHtmlEmbedNode is now a test-only helper; add an Unreleased
  CHANGELOG entry; drop the dangling "arbitrary HTML embed" planning-doc ref.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

AGENTS.md

@@ -280,4 +280,4 @@ The git tag is the source of truth for the displayed version (UI reads `git desc
 
 ## Planning docs
 
-`docs/*.md` hold design plans for in-progress / planned features (mobile app, offline sync, RAG improvements, voice dictation, arbitrary HTML embed). `docs/backlog/*.md` track known issues / follow-ups (e.g. AI-chat review follow-ups). Consult the relevant plan before working on one of those areas.
+`docs/*.md` hold design plans for in-progress / planned features (mobile app, offline sync, RAG improvements, voice dictation). `docs/backlog/*.md` track known issues / follow-ups (e.g. AI-chat review follow-ups). Consult the relevant plan before working on one of those areas.