docs: add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing to Agentree
+
+## Workflow
+
+All changes start from an issue.
+
+1. Open or find an issue describing the work
+2. Create a branch from `main`:
+   ```
+   feat/issue-N-short-description
+   fix/issue-N-short-description
+   chore/issue-N-short-description
+   docs/issue-N-short-description
+   ```
+3. Make changes and commit
+4. Open a PR with `Closes #N` in the description
+5. CI must pass before merge
+
+## Commit style
+
+```
+<type>: <summary>
+```
+
+Types: `feat` / `fix` / `chore` / `docs` / `test` / `refactor`
+
+## Local setup
+
+```bash
+pnpm install
+
+# Configure opencode
+cp .env.opencode.example .env.opencode
+# Edit .env.opencode — set OPENCODE_SERVER_PASSWORD
+
+# Start opencode
+docker compose --env-file .env.opencode -f docker-compose.opencode.yml up -d
+
+# Run DB migrations
+pnpm run db:migrate
+
+# Start dev server
+pnpm run dev
+```
+
+| Service | URL |
+|---------|-----|
+| Agentree UI | http://localhost:5174 |
+| Agentree API | http://localhost:3001 |
+| opencode | http://localhost:6543 |
+
+## Tests
+
+```bash
+pnpm test                                          # unit tests (vitest)
+pnpm exec tsc --noEmit                             # server type check
+pnpm exec tsc --noEmit -p tsconfig.client.json    # client type check
+```
+
+All three must pass before opening a PR. The CI workflow runs them automatically on every PR targeting `main`.
+
+## Schema changes
+
+After editing `src/server/db/schema.ts`:
+
+```bash
+pnpm run db:generate   # generate new migration
+pnpm run db:migrate    # apply migration locally
+```
+
+Commit both the schema file and the generated migration files.