chore: add pull request template

[sentomk]

Summary

Add a pull request template and document the protected main development workflow.

Changes

• Added .github/PULL_REQUEST_TEMPLATE.md with standard sections for summary, changes, tests, risk, and checklist.
• Documented that main is protected and changes should go through pull requests.
• Added branch naming conventions for feature, fix, docs, test, and chore work.
• Added recommended git worktree commands for concurrent local development.
• Noted that PR titles should follow the same style as commit subjects.

Tests

• Not run locally; documentation/template-only change.

Risk

Low. This only affects contribution guidance and the default PR description template.

Checklist

• Tests added or updated when behavior changed
• Documentation updated when public API or behavior changed
• Commit messages follow CONTRIBUTING.md
• PR targets main and passes required CI

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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