feat(mcp): expose thread search

[juliusmarminge]

Problem

Agents can read known threads but cannot search durable thread content and then navigate directly to a matching visible message.

Change

Expose t3_thread_search as a read-only MCP tool over the bounded projection query from #8721. The tool:

• defaults to the calling thread and current project scope
• supports an optional same-project thread filter and archived opt-in
• returns bounded snippets, source and storage origin, live pagination state, and nullable V2 read anchors
• lets t3_thread_read.anchor start inclusively at the matching visible source/message identity
• maps typed input errors to invalid_request and persistence failures to fixed structural errors without exposing SQL details
• remains available to every provider, including Claude's read-only allowlist
• keeps the merged t3_thread_update registration and metadata behavior intact

Behavior

Search is read-only and never imports transcripts, acknowledges child delivery, or returns full messages. An anchor that is foreign, stale, rolled back, or no longer visible fails instead of falling back to a raw ordinal.

Validation

• vp test run apps/server/src/mcp/OrchestratorMcpToolkit.integration.test.ts apps/server/src/mcp/toolkits/worktree/registration.test.ts packages/contracts/src/orchestratorMcp.test.ts packages/shared/src/t3McpToolPresentation.test.ts (15/15)
• production MCP registration with a real SQLite/V2 search hit to anchored-read round trip
• root-object tool schemas, current-project rejection, bounded astral-Unicode result encoding, and provider presentation/allowlist coverage
• scoped t3, @t3tools/contracts, and @t3tools/shared typechecks
• targeted lint, format, and git diff --check

Dependency

Upper member of native stack #8785. Depends on #8721.

Implemented by GPT-5.6-Sol via Codex in T3 Code.

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[macroscopeapp]

Reviewed the new ThreadSearchMcpService and its call sites against the Effect service conventions. One violation found: a standalone ThreadSearchMcpServiceShape interface instead of an inline Context.Service interface. Imports, make/layer naming, dependency acquisition (yield* ThreadManagementService / yield* ProjectionSnapshotQuery), and the test-only Layer.mock seams all look consistent with the conventions.

^{Posted via Macroscope — Effect Service Conventions}

[github-actions]

Thread transfer impact

⚠️ The latest CI run did not produce a thread transfer result for 70a74a5.

This comment will update automatically after the next completed run.

[macroscopeapp]

Approvability

Verdict: Approved at 3430825

Macroscope's review found this PR approvable — This adds a bounded, read-only MCP search tool around the existing projection query, with capability/project scoping, strict result limits, and coverage for contracts, integration, and registration behavior. Existing thread-read behavior and stored data remain unchanged unless the new search/anchor path is explicitly used.

No code changes detected at 70a74a5. Prior analysis still applies.

^{You can add or adjust custom eligibility rules. Learn more.}

[juliusmarminge]

Superseded as the active MCP proposal by #10561 in the rebuilt MCP stack, following the maintainer request to review thin service wrappers separately from service hardening.

This is not a full feature-parity replacement. This filters the global top results, so a project can underfill the requested limit. It is not exhaustive or paginated. Project-first SQL pagination and search-service correctness changes belong in a separate service PR.

Closing this mixed proposal without merging. The remote branch agents/mcp-thread-search/tool and exact head 70a74a55ba4420d213fc2a56e2f627c8bd4d6b96 are retained so the service changes can be extracted for separate review later.