docs: add CONTRIBUTING.md for open-source contributors

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: GrapeInTheTree

CONTRIBUTING.md

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+# Contributing to Pocket Claude
+
+Thanks for your interest! Here's how to get started.
+
+## Development Setup
+
+```bash
+git clone https://github.com/GrapeInTheTree/pocket-claude.git
+cd pocket-claude
+cp .env.example .env    # fill in your Telegram token and chat ID
+go mod download
+```
+
+## Workflow
+
+1. Fork the repo
+2. Create a branch (`git checkout -b feature/my-feature`)
+3. Make changes
+4. Run checks: `make ci` (fmt + vet + build + test with race detector)
+5. Commit with a clear message
+6. Open a pull request
+
+## Code Style
+
+- Follow standard Go conventions (`gofmt`, `go vet`)
+- Keep functions short and focused
+- Add tests for new functionality
+- Use `any` instead of `interface{}`
+
+## Project Structure
+
+```
+cmd/pocket-claude/     Entry point
+internal/
+  bot/                 Telegram listener, commands
+  claude/              CLI execution, session tracking
+  config/              Environment config, logger
+  project/             Multi-project management
+  store/               JSON persistence
+  worker/              Message queue, background pool, ralph loop
+```
+
+## Running Tests
+
+```bash
+make test        # all tests
+make test-race   # with race detector
+make ci          # full pipeline (fmt + vet + build + test)
+```
+
+## Reporting Issues
+
+- Bug reports: include logs (`bot.log`) with sensitive data redacted
+- Feature requests: describe the use case, not just the solution
+
+## License
+
+By contributing, you agree that your contributions will be licensed under the MIT License.