fix(markdown): persist conflict-safe updates

[George Ng (GeorgeNgMsft)]

Stack

Layer 3 of the replacement stack for #2940.

• Base: main ([Markdown] create documents in workspace #2966 and [Markdown Agent] Apply document edits deterministically #2967 are merged)
• Parent layer: [Markdown Agent] Apply document edits deterministically #2967 (merged)

Included

• Persists headless Markdown updates directly to the authorized workspace file.
• Makes streaming completion idempotent by recognizing the already-produced revision instead of applying operations twice.
• Carries immutable binding identity and SHA-256 revision checks across read/apply.
• Rejects stale tokens, rebound root/path/file identities, revision conflicts, and roots replaced by symlinks or junctions.
• Adds request IDs to concurrent document read/apply IPC and correlates responses.
• Keeps the view service server-authoritative for update persistence even when no browser/SSE client is connected.

Deferred to browser layer

• Browser document-manager.ts rewrite and serializer/autosave behavior.
• Primary/secondary SSE lifecycle, browser synchronization/promotion UX, and reopening the same file while preserving browser binding state.
• Broader browser snapshot adoption and loopback binding behavior.

Validation

Historical results from before the rebase (not rerun as full build/Jest validation):

• @typeagent/markdown-agent TypeScript and frontend build
• Focused Jest suites: 44 tests passed across Markdown persistence, operation engine, path policy, creation path policy, and action handler coverage
• Prettier changed-file check
• Git diff check
• Repo lint ratchet

After rebasing onto f3e4308ea2cd722a2905eceac3f2a1e61d19a420: 23 focused Node integration smoke checks passed using the actual handler, persistence module, and operation engine with simulated translator/view IPC; TypeScript 5.4.5 syntax parsing and Git diff checks passed. Full build/Jest, semantic typechecking, and repository ratchets were not rerun: this worktree has no installed dependencies (fluid-build missing). Pinned Prettier 3.5.3 was unavailable offline.

Complete layer diff against the new main base: 992 insertions, 532 deletions (1,524 changed lines), across 6 files.