ci: validate PR title against Conventional Commits

[bluetoothbot]

What

Add a GitHub Actions workflow that validates the pull request title against the Conventional Commits spec.

Why

GitHub uses the PR title — not the first commit's subject — as the commit message when "Squash and merge" is used. The existing commitlint job in ci.yml only lints individual commit messages, so a malformed PR title still produces an invalid commit on main even when every commit on the branch passes. Fixes #531.

How

• New workflow .github/workflows/pr-title.yml runs amannn/action-semantic-pull-request@v6.1.1 (pinned to SHA, matching the rest of the repo).
• Uses pull_request_target so external/fork PRs can be validated safely (only PR metadata is read; permissions: pull-requests: read).
• Types mirror @commitlint/config-conventional (build, chore, ci, docs, feat, fix, perf, refactor, revert, style, test).
• subjectPattern: ^(?![A-Z]).+$ enforces lowercase first letter to match commitlint's subject-case rule.
• Triggers on opened / edited / synchronize / reopened so the check stays current as the title is edited.

Testing

• python3 -c \"import yaml; yaml.safe_load(...)\" confirms the workflow file parses.
• Action SHA verified against the upstream v6.1.1 tag.
• Regex sanity-checked against representative PR titles (lowercase pass, capitalized fail).
• Full end-to-end validation will happen on this very PR — the new workflow will run against this title, which itself follows the conventional-commits format.

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Quality Report

Changes: 1 file changed, 52 insertions(+)

Code scan: clean

Tests: failed ([Errno 13] Permission denied: 'pytest')

Branch hygiene: clean

Generated by Kōan post-mission quality pipeline

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.06%. Comparing base (7edf99f) to head (4c0a536).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #635      +/-   ##
==========================================
- Coverage   86.12%   86.06%   -0.06%     
==========================================
  Files          29       29              
  Lines        3488     3488              
  Branches      602      602              
==========================================
- Hits         3004     3002       -2     
- Misses        299      300       +1     
- Partials      185      186       +1     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 6 untouched benchmarks

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_{Comparing koan/pr-title-lint (4c0a536) with main (7edf99f)¹}

Footnotes

1. No successful run was found on main (bce3ac8) during the generation of this report, so 7edf99f was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[Copilot]

Pull request overview

Adds a dedicated GitHub Actions workflow to lint pull request titles against the Conventional Commits format, ensuring squash-merge commit subjects remain valid even when individual commits already pass commitlint.

Changes:

• Introduces .github/workflows/pr-title.yml to validate PR titles using amannn/action-semantic-pull-request (pinned to a commit SHA).
• Runs on pull_request_target with restricted permissions (pull-requests: read) to safely validate titles for forked PRs.
• Enforces a lowercase-first-character subject convention via subjectPattern.

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