fix(markdown): restore document creation and updates

[George Ng (GeorgeNgMsft)]

Summary

• restore Markdown document creation and updates without requiring a connected browser
• preserve initial content for compound create-and-fill requests and avoid overwriting existing non-empty files
• save documents under the host-authorized workspace root, with safe relative subdirectories and traversal/symlink/junction protections
• propagate the request working directory through CLI, dispatcher ActionContext, and out-of-process agent RPC
• return the real filesystem path and a loopback editor link for every created/opened document
• unify browser and headless Markdown operation semantics, including formatting, serializer-based persistence, revisions, binding identities, restart recovery, and multi-tab primary handoff
• remove the unsafe unused /file/load rebinding path and evict stale collaboration rooms
• preserve typed Azure model configuration while deferring LLM initialization until an update actually needs it

Validation

• affected Markdown, RPC, dispatcher, CLI, and server builds pass
• 103 Markdown tests pass across 9 suites
• 38 agent RPC tests pass across 3 suites
• focused dispatcher ActionContext tests pass
• complexity and lint ratchets pass; changed files are Prettier-clean
• real CLI compound request executed markdown.createDocument, wrote exact content at the repository root, appeared in git status, and returned an editor URL that responded successfully
• completed two principal-engineer adversarial review rounds plus a narrow closure review; all high-confidence findings were addressed

[George Ng (GeorgeNgMsft)]

Separating this out into multiple smaller PRs

[George Ng (GeorgeNgMsft)]

Superseded by native stack #2970, split bottom-to-top for reviewability:

1. [Markdown] create documents in workspace #2966 - workspace-safe document creation
2. [Markdown Agent] Apply document edits deterministically #2967 - deterministic document edit operations
3. fix(markdown): persist conflict-safe updates #2968 - conflict-safe durable/headless persistence
4. fix(markdown): secure browser document bindings #2969 - secure browser binding and synchronization server
5. fix(markdown): persist browser editor changes #2971 - browser editor persistence and reconciliation

The final stack was validated with the Markdown build, all 8 Markdown test suites (64 tests), Prettier, lint/complexity/circular/debt gates, and exact branch ancestry checks.