feat(mcp): target thread launches by project

[juliusmarminge]

Problem

Creating a project over MCP is incomplete when a new top-level thread cannot be launched there, and existing thread tools can only resolve targets in the caller's current project.

Change

• Add optional projectId and typed root, existing-worktree, or new-worktree launch strategies to create_threads and t3_thread_start.
• Route new threads through ThreadLaunchService so provisioning, setup scripts, receipts, provenance, attachments, and idempotency follow the application workflow.
• Authorize and persist the target project when recording a created thread on the parent timeline.
• Canonicalize existing-worktree paths and verify Git repository identity before launch.
• Treat root and existing-worktree branch values as expected-branch constraints and persist the checkout's actual branch.
• Add optional same-environment project selection to thread list, read, send, wait, and interrupt.
• Advertise project targeting and workspace launch strategies through orchestration capabilities.

Behavior

Omitted project selection keeps the current project. Omitted workspace selection reuses the caller checkout only in that project; another selected project starts at its configured root and never inherits the caller's worktree path. Existing-worktree launches accept canonical or symlink paths only when they resolve to a Git worktree root of the selected repository. Root launches preserve configured repository subdirectory roots, and root/existing-worktree launches never switch branches. Provider/model/runtime settings still inherit from the caller, and runtime/interaction permission ceilings remain enforced. No arbitrary environment selection is accepted.

Validation

• Real MCP, ThreadLaunchService, and orchestrator integration covering a recorded cross-project launch.
• Real Git repository tests covering sibling worktrees, canonical symlinks, nested project roots, unrelated repositories, plain directories, and root/existing branch mismatches.
• Focused project-targeting, activity, registration, thread-management, and contract tests: 35 passed.
• @t3tools/contracts and server targeted typechecks; targeted lint for changed orchestrator MCP and contract files.

Dependency

Depends on #8677 for project contracts, persistence plumbing, and ProjectService-backed selection. It is the upper layer of native stack #8710, whose trunk is the immutable agents/mcp-controls/base-490318a. Cross-project organization remains a coordinated follow-up on the same resolver and is intentionally not implemented here.

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

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[github-actions]

Thread transfer impact

⚠️ The latest CI run did not produce a thread transfer result for f17378f.

This comment will update automatically after the next completed run.

[macroscopeapp]

Effect service conventions review: two import-namespace deviations around the new VcsDriverRegistry dependency. Everything else (environment-based dependency acquisition in make, the updated layer requirement type, ThreadLaunchService consumption, and the test seams) follows the conventions.

^{Posted via Macroscope — Effect Service Conventions}

[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This PR adds cross-project thread access and workspace launch workflows, including Git worktree provisioning, setup execution, branch validation, and a new orchestration authorization boundary. It also routes existing thread creation through a materially different launch path, creating broader runtime impact than a small optional API addition.

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[cursor]

Cursor Bugbot has reviewed your changes using high effort and found 1 potential issue.

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