feat(mcp): expand MCP toolset (read tools + git write tools with merge threshold)

[guoyongchang]

Overview

Expands the MCP server toolset and fixes two P0 framework bugs.

• P0 fixes: single CallTool dispatcher with an aggregated handler map; capability level is now read from mcp.json (default details instead of core).
• 9 new tools:
  • Read: get_active_workspace, list_projects, get_branch_status
  • Write: sync_base, pull, push, commit_and_push, merge_to_test, merge_to_base
• Merge threshold gate: merge_to_test / merge_to_base are guarded by an ahead-count threshold (evaluateMergeGate), with hardened input validation and errorResult handling.
• Backend start_mcp_server now preserves an existing user-set capability_level (defaulting to details) instead of overwriting it with core.
• Transport extended with merge/sync/pull, test_branch diff, and write-operation timeouts.
• Root vitest now excludes packages/** (MCP uses node:test, not vitest).

Trust boundary

The MCP HTTP server binds to 127.0.0.1:42819 only and has no token auth — the trust boundary is loopback-only (any local process can call it). Documented in packages/mcp/README.md.

Verification

• cargo check ✓, cargo clippy -- -D warnings ✓
• cd packages/mcp && npm test ✓ (9 passed)
• npm run build ✓, node scripts/check-i18n.mjs ✓ (798 keys), npm run contracts ✓ (in sync), npx vitest run ✓ (74 passed)

Known pre-existing bug (not introduced here)

• git_switch_branch payload shape does not match the backend handler — out of scope for this PR.

Plan

docs/superpowers/plans/2026-06-20-mcp-expand-tools.md