📝 docs: add ADR for staged-only Git hooks formatting

- Add ADR-0012 documenting the decision to format only staged Go files
- Update ADR README.md with new entry
- Document rationale, alternatives, and verification results
- Include future considerations for monitoring and CI/CD integration

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

.githooks/README.md

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+# Git Hooks for DanceLessonsCoach
+
+This directory contains Git hooks for the DanceLessonsCoach project.
+
+## Available Hooks
+
+### pre-commit
+- **Location**: `.git/hooks/pre-commit`
+- **Purpose**: Automatically runs `go mod tidy` and `go fmt` before commits
+- **Features**:
+  - Runs `go mod tidy` to clean up dependencies
+  - Automatically adds modified `go.mod` and `go.sum` to commit
+  - Runs `go fmt` on all Go files (excluding vendor and .git directories)
+  - Automatically adds formatted files to commit
+  - Only runs if in a Go project (checks for `go.mod`)
+
+## Installation
+
+The pre-commit hook is already installed and executable. No additional setup required.
+
+## Usage
+
+The hooks run automatically when you commit:
+
+```bash
+git add .
+git commit -m "Your commit message"
+```
+
+The hook will:
+1. Run `go mod tidy`
+2. Run `go fmt` on all Go files
+3. Add any modified files to your commit
+4. Allow the commit to proceed if successful
+
+## Customization
+
+To modify the hooks:
+1. Edit the hook file in `.git/hooks/`
+2. Make it executable: `chmod +x .git/hooks/hook-name`
+
+## Disabling Hooks
+
+To temporarily disable hooks:
+
+```bash
+# Skip pre-commit hook for one commit
+git commit --no-verify -m "Your commit message"
+```
+
+## Best Practices
+
+- Let the hooks run automatically - they ensure code quality
+- The hooks only modify files that need changes
+- All changes are added to your commit automatically
+- Hooks run quickly and prevent common issues
+
+## Troubleshooting
+
+If a hook fails:
+- Check the error message
+- Fix the issue manually
+- Commit again
+
+Common issues:
+- `go mod tidy` fails: Check your Go module dependencies
+- `go fmt` fails: Check for syntax errors in your Go code