chore: add commitlint checks for commits and PR titles

[mandaiy]

Summary

Add commitlint enforcement for commit messages and PR titles to support squash-merge workflow.

Changes

• Added commitlint.config.cjs with Conventional Commits rules.
• Added commitlint commit-msg hook to .pre-commit-config.yaml.
• Integrated PR title lint job into .github/workflows/ci.yml.
• Updated docs in docs/repository_structure.md and docs/pull_request_template.md.
• Excluded Markdown files from trailing-whitespace pre-commit hook.

Testing

• make format
• make lint
• make test

Checklist

• I updated relevant documentation under docs/.
• PR title follows Conventional Commits (e.g., feat: ..., fix: ...).
• I verified that the changes are backward compatible, or documented breaking changes.
• I confirmed this PR is ready for review.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Load repository config before running PR title lint

The lint-pr-title job runs npx commitlint without ever checking out the repository, so commitlint.config.cjs is not present in that job’s workspace and the PR title is not validated against this repo’s configured Conventional Commits rules (and can fail outright depending on commitlint defaults). This affects every PR event handled by this workflow.

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[chatgpt-codex-connector]

Provision commitlint packages in commit-msg hook

The new commit-msg hook invokes npx ... commitlint --edit via language: system, but the repository does not declare Node dependencies and commitlint.config.cjs extends @commitlint/config-conventional; on a clean machine this hook depends on globally preinstalled packages and will fail for contributors who only installed the repo’s Python toolchain. This makes normal commits fail in common local setups.

Useful? React with 👍 / 👎.