feat(mcp): attach to live Yjs collaboration rooms (co-edit + attributed tracked changes)

[jacobjove]

Closes #3568.

Opened as a draft alongside #3568.

What this adds

A collaboration-attach path for apps/mcp, alongside the existing file-open path, so an MCP client can join a live Yjs room instead of only round-tripping a local .docx. The motivating use case is agent-assisted review: suggest tracked redlines into the document a human has open, attributed to a named reviewer, for accept/reject.

Three capabilities, all kept in the MCP layer — no super-editor core edits:

1. superdoc_attach({ ws_url, document_id, token?, user? }) — connects to a Yjs room over WebSocket, awaits initial sync, returns a session_id usable with every existing tool. Like superdoc_open, it creates a session rather than consuming one.
2. Tracked-change attribution. user: { id?, name?, email? } is threaded into the headless Editor config. Without it, superdoc_mutations({ changeMode: "tracked" }) over an attach throws forceTrackChanges requires a user to be configured on the editor instance (the gate reads exactly this.options.user). The file-open path already defaulted a user; attach was the only path missing the seam. Caller-supplied so changes attribute to the real reviewer, not a hardcoded identity.
3. Collab-aware save export. A joiner editor is built with no docx source (content arrives via the Yjs fragment), so convertedXml lacks the base OOXML parts and exportDocx throws on the first unguarded deref. Fix: seed the blank-docx template (Editor.loadXmlData(blankDocxBytes, true)) so SuperConverter populates the standard parts. With a ydoc present, #createInitialState only uses the seeded content as export scaffolding — Yjs still drives the live body.

Commits

• feat(mcp): collab attach + save export for live Yjs rooms
• feat(mcp): attach can configure a tracked-change user
• test(mcp): cover superdoc_attach in protocol tool-list assertions

Dependencies added: yjs, y-websocket (via catalog:), ws + @types/ws.

Testing

• New apps/mcp/src/__tests__/collab-export.test.ts (joiner-editor export → valid PK-zip with word/document.xml + word/styles.xml + word/_rels/document.xml.rels) and collab-attach-user.test.ts (asserts editor.options.user is set with a user, null without).
• protocol.test.ts updated to include superdoc_attach in the tool-list / action-enum / session_id assertions (it's a session-creating lifecycle tool like superdoc_open).
• Full apps/mcp suite green: 38 pass / 0 fail (pnpm run build:superdoc then pnpm --prefix apps/mcp run test, matching ci-mcp.yml).
• Verified end-to-end against a live collab server: attach with user → tracked mutation succeeds (no "requires a user") → superdoc_track_changes list shows the change attributed to the supplied author → save exports a valid OOXML file with the live-room body intact.

Note for reviewers (latent, left untouched here)

Editor.exportDocx() wraps its whole body in try { … } catch (e) { this.emit('exception', …); console.error(e); } with no return in the catch, so on any export error it silently returns undefined and the real TypeError only reaches stderr. For a collab joiner the throw originates earlier (SuperConverter header/footer export with empty convertedXml). Seeding the template avoids the throw; I left the swallowing catch alone since it's on the shared export path, but it masks the root cause for anyone who hits it. Flagged in #3568.

Checklist

• Changes focused (one feature)
• Tests added
• Suite passes locally (pnpm --prefix apps/mcp run test, after build:superdoc)
• Formatted (prettier --check clean on changed files)
• Conventional Commits
• PR links the related issue (MCP: attach to live Yjs collaboration rooms (co-edit + attributed tracked changes) #3568)