Add MCP harness regression guard

[GRAMMAC1]

Summary

• add an MCP harness regression baseline and verification flow
• keep benchmark:mcp as the report/artifact generation command while sharing the harness runner
• add pnpm verify to run check, lint, typecheck, tests, and MCP harness regression checks
• update AGENTS.md so agents run pnpm verify before finishing non-trivial changes
• add a GitHub Actions PR workflow that runs pnpm verify against master

Verification

• pnpm benchmark:mcp
• pnpm harness:mcp
• pnpm verify