feat(orchestrator): Start threads in known projects

[mwolson]

What Changed

Agents can now start top-level threads in another project that T3 Code already knows. create_threads accepts a projectDirectory per entry, an absolute or ~/ path that must match a registered project root; the new thread starts at that root with no inherited branch or worktree. Omitting it keeps the caller's checkout, and delegated children always stay in the caller's project. t3_thread_launch already accepts a projectId.

The calling thread records each thread it creates this way in its timeline, including threads in another project. Retrying a request with the same clientRequestId returns the same threads, and fails if one of them has since been deleted.

Why

Parent agents could only start ordinary threads in their own project. Upstream's explicit MCP thread and project targets (pingdotgg#15219) already let a caller read, send to, wait on and list threads in other projects, so this layer now only adds choosing the project at creation and recording the created thread.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes