chore: add pinned messcript quality gate

[jonbaldie]

Closes #28.

Summary

• Pin messcript to commit 4fe47bd0f15675206aedd0f22ae5eff7aeb01707.
• Add a Node wrapper that clones, checks out, installs, builds, and executes that exact revision.
• Add named commands for all eight production units plus the aggregate production surface.
• Document scope, policy, default thresholds, rule identities, and exit-status behavior.
• Carry forward the requested Serena tooling refresh, now shipped on main by ci: migrate to GitHub Actions #39.

Validation

• npm run quality:unit -- configuration
• npm run quality:unit -- router
• npm run quality:unit -- queue-manager
• npm run quality:unit -- rate-limiter
• node --check scripts/messcript.mjs
• deno fmt --check scripts/messcript.mjs
• deno lint scripts/messcript.mjs
• deno test --allow-all (177 passed)
• Verified processing errors return exit 1 and findings return exit 2.
• GitHub Actions runs Test → Mutasaurus → Stryker against the updated branch.