chore: add CONTRIBUTING.md with contributor guidelines

[RyanBayne]

Summary

There is no CONTRIBUTING.md file. GitHub surfaces this automatically to contributors when they open a PR or issue, making it a high-value, low-effort addition.

What to include

• How to set up the dev environment (npm install, npm run dev)
• Branch and commit message conventions
• PR checklist expectations
• Where to ask questions (Discussions)
• A pointer to the wiki developer source

Acceptance Criteria

• CONTRIBUTING.md exists at the repository root
• Covers environment setup, branching, commits, and PR process
• Links to relevant wiki or discussion threads