Add CI/CD pipeline and SDLC docs

CONTRIBUTING.md

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+# Contributing to ShellMate
+
+Thanks for your interest in contributing! 🎉
+
+## Branch Strategy
+
+```
+feature/* ──→ develop ──→ master
+                │            │
+             staging      production
+```
+
+- **`develop`** — Default branch, all PRs target here
+- **`master`** — Production releases only
+- **`feature/*`** — Feature branches off develop
+
+## Development Flow
+
+1. Fork the repo
+2. Create a feature branch from `develop`:
+   ```bash
+   git checkout develop
+   git pull origin develop
+   git checkout -b feature/my-feature
+   ```
+3. Make your changes
+4. Run tests: `pytest tests/ -v`
+5. Run linting: `ruff check src/`
+6. Push and create a PR to `develop`
+
+## Code Style
+
+- Python 3.11+
+- Type hints required
+- Ruff for linting
+- 100 char line limit
+
+## Commit Messages
+
+Use conventional commits:
+- `feat:` New feature
+- `fix:` Bug fix
+- `docs:` Documentation
+- `refactor:` Code refactor
+- `test:` Tests
+- `chore:` Maintenance
+
+## Release Process
+
+1. PRs merged to `develop` deploy to staging
+2. When ready, merge `develop` → `master`
+3. Master deploys to production automatically