docs: add CONTRIBUTING.md

[StatPan]

Closes #2

What

Adds CONTRIBUTING.md documenting the issue → branch → PR workflow, branch naming convention, commit style, local setup steps, and test commands.

Why

The repo is public with CI enforced. Contributors and maintainers need a canonical reference for how changes flow into the project. This document is also the pattern we follow for all subsequent issues (#3, #4, ...).

Test plan

• pnpm test passes
• pnpm exec tsc --noEmit passes
• pnpm exec tsc --noEmit -p tsconfig.client.json passes
• Manually verified content accuracy against actual repo setup

[gemini-code-assist]

Code Review

This pull request introduces a new CONTRIBUTING.md file, providing comprehensive guidelines for contributing to the Agentree project. It covers the workflow, commit style, local setup instructions, testing procedures, and steps for managing schema changes. There is no feedback to provide from the review comments.