Add CONTRIBUTING.md

[Ryan4n6]

Context

The repo needs a CONTRIBUTING.md to lower the barrier for new contributors.

Should Cover

• How to test changes locally (install commands, run hooks manually)
• How to add a new memory template
• How to add a new slash command
• PR conventions (branch naming, commit messages)
• How to report bugs vs request features
• Code of conduct (or link to a standard one)

Good First Issue

This is a documentation-only task. Read the existing docs to understand the system, then write the contributor guide.

[monssefbaakka]

I've opened a PR and added the CONTRIBUTING.md file addressing all the points mentioned! Let me know if you'd like me to make any changes or adjustments.