feat(mcp): let agents change thread workspaces - #8680

Agents need a safe way to move an existing thread to a branch, project root, existing checkout, or new worktree while keeping durable thread metadata aligned with actual Git state.

This dependent layer adds t3_thread_checkout over the existing VCS, V2 command, provider-session, and continuation workflows. It verifies requested refs, canonical repository/worktree identity, dirty state, active and shared ownership, and the final checked-out state before committing the binding. Physical workspace admission covers aliases and nested project roots. New-worktree handoff rechecks ownership after creation and retains any checkout whose cleanup ownership becomes uncertain.

Pre-mutation reads remain cancellable. Once mutation starts, the narrow Git/binding/rollback section preserves atomicity and truthful partial-failure outcomes. A successful move ends the current provider turn; continuationPrompt schedules the replacement turn in the selected workspace. Status broadcasting is refreshed after committed changes without allowing refresh failure to rewrite the completed result.

Focused validation:

• 314 tests across the combined inventory/checkout stack, including real Git ref handling, cancellation, concurrent ownership, rollback retention, provider detach/continuation, contracts, registration, and presentation
• vp run --filter t3 typecheck
• vp run --filter @t3tools/contracts typecheck
• vp run --filter @t3tools/shared typecheck
• vp run --filter @t3tools/client-runtime typecheck
• Targeted vp lint, vp fmt --check, and git diff --check

Depends on #8685 for canonical bounded workspace discovery. Native stack: #8685 → #8680, rooted on t3code/codex-turn-mapping at 415ed0f73b97f1655b6282492f81d0b2bba3a9cc.

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