Infrastructure: add .github directory (templates, PR template, CI/CD, docs)

[rosspeili]

Summary

Add a standard .github/ layout so contributions, reviews, and quality checks are consistent. The repository currently has no .github/ directory; governance lives in root files such as CONTRIBUTING.md and SECURITY.md.

This epic has four sub-issues. Implement and close them in a sensible order (templates and PR template first, then CI/CD, then documentation).

Scope

Area	Deliverable
Issue templates	.github/ISSUE_TEMPLATE/ plus label strategy (see sub-issue)
Pull requests	.github/pull_request_template.md (simple, checklist-oriented)
CI/CD	GitHub Actions for format, lint, and related quality gates
Documentation	Update CONTRIBUTING.md (and README.md cross-links if needed)

Labels (current state)

The repo only has GitHub’s default labels (bug, documentation, duplicate, enhancement, good first issue, help wanted, invalid, question, wontfix). Use those on this epic and sub-issues until the issue-templates work adds project-specific labels and ties them to templates.

Out of scope (separate epic later)

• Refactoring root layout (modules/ vs top-level entry) — file after templates exist.

Acceptance criteria

• .github/ exists and is referenced from contribution docs
• All four sub-issues below are completed or explicitly deferred with rationale
• No secrets, personal kun.py, or enrollment assets committed as part of this work

Sub-issues

• Issue templates and label strategy (.github/ISSUE_TEMPLATE/)
• Pull request template
• CI/CD workflows (Black, Flake8, Pylint, and related config)
• Update CONTRIBUTING.md for the new GitHub workflow