Add regression protection checks for PRs

[luckeyfaraday]

Summary

• add a PR workflow that runs backend tests, client tests, client build, and an explicit regression suite
• add permanent regression test locations for backend and client bugs
• add local regression runners and PR checklist expectations for bug-fix coverage

Testing

• python3 scripts/run_regression_checks.py
• python3 -m pytest
• npm run test:chat
• npm run build

Regression Protection

• If this fixes a bug, I added or updated a regression test that would fail on the old behavior.
• The regression test is linked to the issue, PR, or incident it protects.
• If no regression test was added for a bug fix, I explained why in this PR.

This PR adds the regression-protection system itself. It creates the permanent locations and CI gate for future bug-specific regression tests; it does not fix a specific product bug.