Docs: update CONTRIBUTING.md for .github workflow

[rosspeili]

Summary

Refresh CONTRIBUTING.md after issue templates, PR template, and CI land so contributors know how to file issues, open PRs, and pass checks.

Updates to include

• Reporting work — which issue template to pick; for now, which default label applies (bug, documentation, enhancement, etc.)
• Future labels — pointer that OPSIIE-specific pastel labels will be added after the templates issue follow-up
• Pull requests — reference .github/pull_request_template.md
• Local quality gates — commands matching CI (Black, Flake8, Pylint, optional pytest)
• Security — no secrets; use kun.example.py; see SECURITY.md

Acceptance criteria

• CONTRIBUTING.md matches what actually shipped in .github/
• Windows (PowerShell) commands for local checks; note Linux/macOS if useful
• Consistent with CODE_OF_CONDUCT.md and SECURITY.md
• No emojis unless the project later standardizes otherwise

Depends on

• Parent: Infrastructure: add .github directory
• Sub-issues: issue templates, PR template, CI/CD

Suggested order

Land after or together with the final .github implementation PR.