feat(mcp): expose project thread launch service

[juliusmarminge]

Part 15/16 of the shared-core and MCP stack. Based on #10564. Next: #10566.

Expose ThreadLaunchService through t3_thread_launch, including a registered project selector, workspace strategy, optional message and pending attachments.

Existing create_threads/t3_thread_start are untouched. Requires a full-access/default caller. Each call is a new launch; failures can leave an accepted partial thread. Preparation and workspace behavior remain in ThreadLaunchService. No new parent-run admission or retry journal.

Attachment claiming and launch execution now go through ThreadMessageIntake, shared with RPC. MCP retains caller defaults, pending-upload policy, and response mapping.

MCP-only rebuild of #8678, preserving attribution to Julius Marminge's original work. Original branches remain available for separate service follow-ups.

Validation: The composed stack passes 94 tests across 12 focused files, including shared core MCP and real MCP/V2 integration, attachment intake, project RPC/service contracts, and client model command selection. Server/contracts/shared/client-runtime typechecks and targeted format/lint/diff checks pass. New behavior coverage lives with the shared operation; no per-tool mock suite was added. Current-head CI is shown below.

Layer size: 3 files, +102/-1. No domain-service production implementation or documentation files change in this MCP layer.

Prepared with Codex in the OpenAI agent runtime.

Note

Add t3_thread_launch MCP tool and handler to ProjectToolkit

• Adds the t3_thread_launch handler in project.ProjectHandlersLive, which validates a full-access caller, accepts pending-upload attachments, applies caller defaults for project/model/modes, and launches a thread through ThreadMessageIntake, returning thread ID and optional initial-run status.
• Adds the project.ThreadLaunchTool schema with bounded message, up to eight attachments, and structured success/failure output; registers it as the first tool in ProjectToolkit.
• Adds a display-name entry ("Launch a project thread") in T3_MCP_TOOLS.
• Risk: launch preparation may continue after acceptance and can leave a created thread on failure; callers must supply only pending-upload attachments or receive an orchestration error.

^{Macroscope summarized 3492b15.}

[github-actions]

Thread transfer impact

⚠️ The latest CI run did not produce a thread transfer result for 3492b15.

This comment will update automatically after the next completed run.

[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This PR adds a new destructive, open-world MCP operation that can create project threads, launch model runs, provision worktrees, and execute setup scripts, including asynchronous preparation after acceptance. Its broad runtime side effects make it unsuitable for automatic approval.

No code changes detected at 3492b15. Prior analysis still applies.

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