Server-side conversion between a file's markdown (the durable source of truth) and its collaborative Yjs document, so the server can own the doc: seed it, project it back to markdown, and let the agent write into it while a user is typing.

Collaborative file editing had two writers with no shared CRDT: copilot edit_content wrote markdown straight to the file while the user typed into an ephemeral, client-seeded Yjs doc. They couldn't reconcile — the agent's edit didn't stream into the editor, and last-writer clobbered. The fix is a server-authoritative Yjs doc both sides write into, with markdown as a projection.

• Parity by construction. The markdown↔ProseMirror step reuses the exact client engine (parseMarkdownToDoc / serializeDocToMarkdown, @tiptap/markdown on the shared extension set) — not a second markdown implementation — so the server can never diverge from what the editor renders. The custom-fidelity constructs (tables, footnotes, raw HTML, sim: mentions) are covered by the same code that covers them in the browser; the round-trip test asserts equivalence.
• Same Yjs binding as the browser. ProseMirror↔Yjs uses @tiptap/y-tiptap (what TipTap's Collaboration extension uses), pinned to the same version and sharing the same prosemirror-model / yjs instances (peer deps) — so the structure the server produces is byte-compatible with the client, targeting the same 'default' fragment.
• Merge, not replace. applyMarkdownToYDoc uses updateYFragment (the primitive ySyncPlugin runs on every keystroke) to apply only the diff, so Yjs reconciles the agent's write with in-flight remote edits. The test proves an agent write and a concurrent remote edit both survive.
• Server-only, DOM via jsdom. The markdown engine builds a (never-mounted) TipTap editor that needs a DOM; on the server it's backed by a single lazily-created jsdom window. Lazy-required so the client bundle never pulls jsdom in.

The realtime relay seeds each room's document from this module over an internal endpoint (buildFileDocSeed → POST /api/internal/file-doc/seed → ensureServerSeed), which let the entire client-seeder subsystem (election / deadlines / triedSeeders / MAX_SEED_ROUNDS / the SEED_REQUEST handshake) be deleted. The client's connect-deadline offline fallback is deliberately kept — it is unrelated to seeding. No feature flag: the cutover is all-at-once.

• Durable persistence. A DB column for the Yjs binary + debounced snapshotting, so a document survives with no collaborators connected instead of being re-seeded from markdown on cold open.
• Copilot into the doc + projection. edit_content calls applyMarkdownToYDoc when a doc is live; a debounced yDocToMarkdown projection keeps the file's markdown current.