docs: expand contributor guide and templates

[huazhuang80-star]

Closes #19.

Summary

• expand CONTRIBUTING.md with prerequisites, setup steps, layer-specific workflows, test commands, code style, conventional commits, and PR checklist
• add GitHub issue templates for bugs, feature requests, and contract issues
• add a pull request template that prompts for issue link, test evidence, risk, rollout notes, and secret checks

Testing

• python -c "exec(''from pathlib import Path\nimport yaml\nfor p in Path(\''.github/ISSUE_TEMPLATE\'').glob(\''*.yml\''):\n data=yaml.safe_load(p.read_text(encoding=\''utf-8\''))\n assert data.get(\''name\''), p\n assert data.get(\''body\''), p\n print(\''OK\'', p)'')"