Add comprehensive pre-commit hook documentation

📚 PRE_COMMIT_HOOK.md:
- Complete guide to the pre-commit hook system
- Explains how the 'ALL TESTS MUST PASS' rule works
- Provides usage examples and troubleshooting
- Documents best practices and benefits
- Includes emergency bypass procedures

✅ Key Features Documented:
- Automatic test execution before commits
- Test blocking for failed tests
- Comprehensive test runner script
- Quality assurance benefits
- Team collaboration advantages

This ensures all developers understand and can effectively use the pre-commit hook system.

PRE_COMMIT_HOOK.md

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+# Pre-Commit Hook System
+
+## Overview
+
+This project now enforces the rule: **ALL TESTS MUST PASS BEFORE COMMITTING**. This is implemented through a Git pre-commit hook that automatically runs tests before allowing any commit.
+
+## What It Does
+
+### ✅ Pre-Commit Hook (`.git/hooks/pre-commit`)
+- **Automatically runs tests** before every `git commit`
+- **Blocks commits** if any tests fail
+- **Provides helpful error messages** and guidance
+- **Can be bypassed** with `--no-verify` (not recommended)
+
+### ✅ Test Runner Script (`scripts/run-tests.sh`)
+- **Comprehensive test suite runner** for manual testing
+- **Runs different test categories** separately
+- **Provides detailed reporting** and success/failure tracking
+- **Helps identify** which specific tests are failing
+
+## How It Works
+
+### 1. Automatic Test Execution
+Every time you run `git commit`, the pre-commit hook:
+1. Runs core tests (main, middleware, logging, errors)
+2. Checks if all tests pass
+3. Allows commit if tests pass
+4. Blocks commit if tests fail
+
+### 2. Test Categories
+- **Core Tests**: Always run in pre-commit hook
+  - `test_main.py` - Main application tests
+  - `test_middleware.py` - Request middleware tests
+  - `test_logging.py` - Logging system tests
+  - `test_errors.py` - Error handling tests
+
+- **Full Test Suite**: Available via `scripts/run-tests.sh`
+  - All test files including API, services, 502 prevention, etc.
+
+## Usage
+
+### Normal Development Workflow
+```bash
+# Make your changes
+git add .
+git commit -m "Your commit message"
+# Tests run automatically - commit only succeeds if tests pass
+```
+
+### Manual Test Running
+```bash
+# Run comprehensive test suite
+./scripts/run-tests.sh
+
+# Run specific test files
+python -m pytest tests/test_main.py -v
+
+# Run all tests
+python -m pytest tests/ -v
+```
+
+### Emergency Bypass (NOT RECOMMENDED)
+```bash
+# Only use in emergencies - bypasses all tests
+git commit --no-verify -m "Emergency commit"
+```
+
+## Benefits
+
+### 🛡️ Quality Assurance
+- **Prevents broken code** from being committed
+- **Enforces test-driven development**
+- **Maintains code quality standards**
+- **Prevents regressions** from being introduced
+
+### 🚀 Development Efficiency
+- **Catches issues early** before they reach the repository
+- **Provides immediate feedback** on code changes
+- **Reduces debugging time** in production
+- **Maintains consistent code quality**
+
+### 📊 Team Collaboration
+- **Ensures all team members** follow the same standards
+- **Prevents "it works on my machine"** issues
+- **Maintains repository stability**
+- **Facilitates code reviews**
+
+## Configuration
+
+### Pre-Commit Hook Location
+- **File**: `.git/hooks/pre-commit`
+- **Permissions**: Executable (`chmod +x`)
+- **Scope**: Project-specific (not shared via git)
+
+### Test Runner Location
+- **File**: `scripts/run-tests.sh`
+- **Permissions**: Executable (`chmod +x`)
+- **Scope**: Project-wide (shared via git)
+
+## Troubleshooting
+
+### Tests Fail in Pre-Commit Hook
+1. **Fix the failing tests** first
+2. **Run tests manually** to verify: `python -m pytest tests/ -v`
+3. **Try committing again**
+
+### Pre-Commit Hook Not Working
+1. **Check permissions**: `ls -la .git/hooks/pre-commit`
+2. **Make executable**: `chmod +x .git/hooks/pre-commit`
+3. **Verify content**: `cat .git/hooks/pre-commit`
+
+### Need to Disable Temporarily
+```bash
+# Rename the hook to disable it
+mv .git/hooks/pre-commit .git/hooks/pre-commit.disabled
+
+# Rename back to enable it
+mv .git/hooks/pre-commit.disabled .git/hooks/pre-commit
+```
+
+## Best Practices
+
+### ✅ Do
+- **Write tests** for all new features
+- **Fix failing tests** before committing
+- **Run full test suite** before major changes
+- **Use descriptive commit messages**
+
+### ❌ Don't
+- **Bypass tests** with `--no-verify` unless absolutely necessary
+- **Commit broken code** even temporarily
+- **Ignore test failures** or warnings
+- **Skip writing tests** for new functionality
+
+## Success Metrics
+
+- ✅ **Zero broken commits** in repository history
+- ✅ **All tests pass** before every commit
+- ✅ **Consistent code quality** across all changes
+- ✅ **Reduced production bugs** due to early detection
+- ✅ **Faster development cycles** with immediate feedback
+
+---
+
+**Remember**: The pre-commit hook is your friend! It helps maintain code quality and prevents issues from reaching production. Embrace it as part of your development workflow.