feat: add governed MCP stdio integration

[linjiaye929]

Summary

• add host-pinned, approval-gated local MCP stdio servers with exact protocol, identity, Tool, and schema verification
• route verified MCP Tools through the existing Registry, preview, hook, Policy, approval, and execution path as extension-originated calls
• bound lifecycle, concurrency, timeouts, results, metadata, and SDK callbacks; prove the real official-SDK stdio path and cross-task cleanup
• add MCP architecture, threat-model, ADR, learning, and resume materials
• make wheel and sdist builds reproducible and reject local state in release archives

Test Plan

• Python 3.12.13: 961 passed, 10 skipped, branch coverage 90.84%
• Python 3.13.14: 961 passed, 10 skipped, branch coverage 90.84%
• Ruff format/check, strict Pyright, Bandit, and pip-audit: passed
• wheel and sdist repeated-build SHA-256 comparison: identical
• wheel and sdist isolated smoke on Python 3.12 and 3.13: passed
• real MCP stdio connect, call, and close smoke for all four artifact/interpreter combinations: 2 passed
• GitHub Actions run 28501386707: quality plus Linux/Windows on Python 3.12/3.13 passed