feat(mcp): Phase 4 — wire readonly worktree tools

[erishforG]

무엇

• Wire MCP worktree_list to return parsec-managed workspaces from state.
• Wire MCP worktree_status to validate a ticket and return the matching workspace.
• Keep mutating worktree MCP tools as stubs for later phases.

왜

Refs #293. This advances the MCP stdio transport from dispatch-only toward real read-only worktree tool behavior while staying inside src/mcp/ and tests/mcp/. @erishforG

변경

• Added repo argument handling through McpContext plus ParsecConfig/WorktreeManager lookup.
• Added deterministic unit tests backed by a temp git repo and .parsec/state.json.
• Renamed the stdio smoke fixture case to reflect the new missing-worktree error behavior.

다음 Phase 힌트

• Add fixture coverage for successful worktree_list/worktree_status through the stdio dispatcher.
• Wire the next read-only tool once response contracts are stable.

리스크

Low. Read-only MCP-only handler wiring; no CLI signature, workflow, or core git mutation changes.

롤백

Revert this PR to return worktree_list and worktree_status to not-implemented MCP stubs.

Test plan

• cargo build --quiet
• cargo fmt --check
• cargo clippy --all-targets -- -D warnings
• cargo test --quiet