ci: enforce regression tests for fix commits (#517)

* ci: enforce regression tests for fix commits

Add a commit-msg hook and CI workflow that require test changes
alongside bug fix commits, ensuring every fix includes a regression
test that would have caught the bug.

- scripts/commit-msg-regression.sh: local git hook (blocks fix commits
  without test changes; exempts static/docs-only; bypass via
  [skip-regression-check] marker)
- .github/workflows/regression-test-check.yml: CI mirror on PRs
  (checks title + commit messages; skip via label)
- scripts/dev-setup.sh: install hook in step 6
- .github/scripts/create-labels.sh: add skip-regression-check label
- CLAUDE.md: document regression test policy

Co-Authored-By: Claude Opus 4.6 <[email protected]>

* fix: address PR review feedback on regression test enforcement

- Use here-strings instead of echo|grep to avoid misinterpreting
  special characters in variables
- Use git diff -W (whole-function context) to detect edits inside
  existing test functions, not just new #[test] attributes
- Honor [skip-regression-check] in commit messages in CI (not just
  the PR label)
- Use git rev-parse --git-path hooks for worktree-safe hook install

[skip-regression-check]

Co-Authored-By: Claude Opus 4.6 <[email protected]>

* Update .github/workflows/regression-test-check.yml

Co-authored-by: Copilot <[email protected]>

---------

Co-authored-by: Claude Opus 4.6 <[email protected]>
Co-authored-by: Copilot <[email protected]>

.github/scripts/create-labels.sh

@@ -62,6 +62,9 @@ create "scope: ci"            "546E7A" "CI/CD workflows"
 create "scope: docs"          "78909C" "Documentation"
 create "scope: dependencies"  "90A4AE" "Dependency updates"
 
+echo "==> Creating workflow labels..."
+create "skip-regression-check" "9E9E9E" "Acknowledged: fix without regression test"
+
 echo "==> Creating contributor labels..."
 create "contributor: new"         "FFF9C4" "First-time contributor"
 create "contributor: regular"     "FFE082" "2-5 merged PRs"