feat(mcp): expose branch-backed workspace discovery

[juliusmarminge]

Part 16/16 of the shared-core and MCP stack. Based on #10565.

Expose GitWorkflowService.listRefs for the calling thread workspace. Existing worktree handoff and status tools stay unchanged.

This discovers branch-backed refs/worktrees only. Detached unbound inventory and switching an existing checkout require a shared service operation and are intentionally not rebuilt inside MCP. The old workspace work is retained for that extraction.

MCP-only rebuild of #8685, #8680, 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: 5 files, +65/-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_worktree_list MCP tool for branch-backed workspace discovery

• Adds a readonly t3_worktree_list tool to the worktree toolkit that lists git refs using the caller's worktree path or the project workspace root.
• The handler checks the worktree capability and project existence, returning structured failures when either is missing; git errors map to an unavailable failure.
• Registers the tool in CLAUDE_READ_ONLY_T3_MCP_ALLOWED_TOOLS and the T3_MCP_TOOLS presentation registry (label "List workspace branches"), and updates the Claude readonly allowlist test to cover the new tool.

^{Macroscope summarized 1dce00c.}

[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This PR adds a new MCP tool for discovering workspace branches and checkout paths, including wiring it into the default Claude read-only tool policy. The behavior is read-only and scoped, but the new capability and policy default change warrant human review.

No code changes detected at 1dce00c. Prior analysis still applies.

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

[github-actions]

Thread transfer impact

⚠️ The latest CI run did not produce a thread transfer result for 1dce00c.

This comment will update automatically after the next completed run.

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