Add CONTRIBUTING.md, issue templates, and PR template (#303)

[aarontrowbridge]

Closes #303

Adds the five pieces of development infrastructure every open-source repo needs:

• CONTRIBUTING.md — repo structure, dev setup, the issue+PR gate, skill authoring, testing tiers, CI/CD overview, and how to get help
• bug_report.yml — structured bug report form with reproduction steps, environment fields, and log output
• feature_request.yml — structured feature request with motivation, proposal, and alternatives
• config.yml — disables blank issues, redirects to Discussions for help
• pull_request_template.md — required issue reference, type of change, verification checklist