feat(mcp): Phase 3 — tools/call dispatch boundary

[erishforG]

무엇

Wire MCP tools/call through a src/mcp handler dispatch boundary and convert placeholder todo!() stubs into structured MCP error envelopes.

왜

Refs #293. The stdio server already accepts tools/call; this phase makes the call path exercise real module handlers without panicking, preparing later PRs to replace stubs one tool at a time.

변경

• Added src/mcp/tools::dispatch and per-tool handler lookup.
• Passed McpContext from the stdio serve loop into JSON-RPC dispatch.
• Returned MCP content envelopes for both tool success and handler errors.
• Replaced todo!() handler stubs with explicit not-implemented errors.

다음 Phase 힌트

Implement the first read-only handler, likely worktree_status or health_check, behind the new dispatch boundary.

리스크

Low. Scoped to src/mcp/; existing CLI command signatures and core git/worktree logic are unchanged.

롤백

Revert this PR to return tools/call to its previous always-not-implemented response.

Test plan

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

@erishforG