feat(mcp): block incoming messages from other threads

[Mjones13]

Threads currently accept unsolicited messages from peer agents. This adds a persistent per-thread block through t3_thread_update with block_thread_messages and allow_thread_messages, with status exposed by thread read/list/configuration tools. Unblocking through MCP is restricted to the recipient itself; direct owner input remains available.

The server rejects new peer messages, attachments, merge-backs, queued-text edits, and question answers before delivery effects. Self messages, task completion notifications, and already accepted work continue normally. Forks and subagent threads start with an independent, unblocked setting.

Stack: #77 → #78 → #83 → #82 → this PR. Base is work/graceful-worktree-start-stack-20261004 at 3646c69a0568def26dbacdf4ace736ecc9d5e447. Do not merge. PR #85 is a separate sibling branch and is not included.

Validation:

• Four focused schema/fork/subagent suites passed: 62 tests.
• Four focused server suites cannot load because the parent stack imports missing V1 orchestration modules from NativeCreationRepository. A temporary path-only repair exposed a removed OrchestrationCommand schema, so that unrelated repair was reverted.
• Server typechecking remains failing on the unfinished parent migration. Contracts typechecking also fails on two unchanged nativeCreation.test.ts references to creationRejectionCode; the changed contract files have no type errors. Targeted lint reports an existing namespace-import error at Orchestrator.ts:1, plus existing warnings. This PR does not claim full integration validation.
• The exact eight-file regression command is recorded as t3-thread-message-blocking in tests/check-groups.json.
• Required formatting of two pre-existing files is isolated in the first commit; the second commit contains the feature, tests, and usage documentation. No UI changes, live installation changes, or merge performed.

Implemented with GPT-6 Astra and GPT-6.1 Sol through the Codex harness in T3 Code.

---

^{Need help on this PR? Tag @codesmith-bot with what you need. Autofix is disabled.}