Contributor guide and project governance

[troyscott]

Description

Open-source project documentation for contributors.

Deliverables

• CONTRIBUTING.md — how to contribute, PR process, code style
• CODE_OF_CONDUCT.md
• Issue and PR templates
• SECURITY.md — how to report vulnerabilities
• Branch strategy documented (main + feature branches)

Acceptance criteria

• CONTRIBUTING.md with clear setup instructions
• Issue templates for bugs, features, and questions
• PR template with checklist
• CODE_OF_CONDUCT.md (Contributor Covenant)
• SECURITY.md

[ethanaurellano]

Can I work on this issue.