docs: add git workflow guidelines

CONTRIBUTING.md

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+# Git Workflow
+
+## Branch Strategy
+
+```
+feature/* → dev → master
+```
+
+| Branch | Purpose |
+|--------|---------|
+| `master` | Production only |
+| `dev` | Integration/testing |
+| `feature/*` | All new work |
+
+## Rules
+
+1. **All work starts on a feature branch** - Never commit to dev/master directly
+2. **Commit early and often** - Working code = committed code
+3. **No uncommitted files** - Working directory must be clean before switching branches
+4. **Always flow forward** - feature → dev → master, never skip
+5. **Reset feature branches after merge** - Keep them synchronized with master
+
+## Workflow
+
+```bash
+# Start work
+git checkout -b feature/name
+
+# Commit working code immediately
+git add .
+git commit -m "feat: description"
+
+# Test on dev
+git checkout dev
+git merge feature/name
+
+# Deploy to production
+git checkout master
+git merge dev
+
+# Clean up
+git checkout feature/name
+git reset --hard master
+```
+
+## Pre-Flight Check
+
+Before switching branches:
+```bash
+git status  # Must be clean
+```