feat(samoid): implement hook execution runtime (#3)

[behrangsa]

Summary

Implement the hook runner that executes actual hook scripts with proper environment setup, error handling, and debugging support as specified in issue #3.

🎯 Acceptance Criteria Complete

All acceptance criteria from issue #3 have been successfully implemented:

• ✅ Execute hook scripts from project root directory
• ✅ Load initialization script from ~/.config/samoid/init.sh
• ✅ Handle SAMOID=0 environment variable to skip execution
• ✅ Support SAMOID=2 for debug mode with script tracing
• ✅ Exit with hook script's exit code
• ✅ Display informative error messages for failed hooks
• ✅ Show "command not found" message when PATH is incorrect
• ✅ Skip execution if hook script file doesn't exist

🏗️ Architecture & Design

Hook Runner Binary

• New samoid-hook binary that serves as the actual Git hook executor
• Added to Cargo.toml with dedicated binary configuration
• Designed to be installed as the Git hook script

Environment Variable Controls

• SAMOID=0: Skip all hook execution (useful for CI/deployment, rebasing)
• SAMOID=1: Normal execution mode (default)
• SAMOID=2: Enable debug mode with detailed script tracing

Dependency Injection Pattern

Follows established codebase patterns for complete test isolation:

• Environment, CommandRunner, FileSystem trait abstractions
• Production implementations: SystemEnvironment, SystemCommandRunner, SystemFileSystem
• Mock implementations with Arc<Mutex<T>> for thread-safe testing

Cross-Platform Support

• Primary path: ~/.config/samoid/init.sh (using XDG_CONFIG_HOME)
• Windows fallback: %USERPROFILE%\.config\samoid\init.sh
• Home directory detection via HOME or USERPROFILE environment variables

🧪 Testing Strategy

Comprehensive Unit Testing

• 30+ Unit Tests: Cover all execution paths and error scenarios
• Mock-Based Testing: Complete isolation from system dependencies
• Error Scenario Coverage: Exit codes, command not found, file system errors
• Argument Handling: Various hook types and argument passing

Integration Testing

• Real Hook Execution: Validated with actual shell scripts
• Environment Mode Testing: Verified skip (SAMOID=0) and debug (SAMOID=2) modes
• Error Propagation: Confirmed proper exit code handling and error messages

Quality Metrics

Following the codebase's high-quality testing standards established in the existing implementation.

📦 Implementation Details

Hook Execution Flow

1. Parse SAMOID environment variable for execution mode
2. Extract hook name from command arguments
3. Load initialization script from ~/.config/samoid/init.sh (if exists)
4. Locate hook script at .samoid/_/{hook_name}
5. Execute hook script with proper environment and arguments
6. Handle errors and propagate exit codes to Git

Error Handling

• Graceful Exit: Silent exit when hook script doesn't exist (normal Git behavior)
• Clear Error Messages: Descriptive failures for script execution errors
• Command Not Found: Special handling for exit code 127 with PATH information
• Debug Information: Detailed tracing in debug mode (SAMOID=2)

Files Modified

• Cargo.toml: Added samoid-hook binary configuration
• src/hook_runner.rs: Complete hook runner implementation with tests

🔗 Dependencies

• Depends on: Core Hook Installation System #1 (Core Hook Installation System) - ✅ Completed
• Depends on: Command Line Interface #2 (Command Line Interface) - ✅ Completed
• Blocks: Robust Error Handling #6 (Error Handling), Cross-Platform Compatibility #7 (Cross-Platform Compatibility)
• Relates to: Comprehensive Git Hooks Support #4 (Git Hooks Support)

📋 Test Plan

• Unit tests pass with comprehensive coverage
• Integration tests validate real hook execution
• Manual testing of all environment modes (SAMOID=0/1/2)
• Error scenario validation (failed hooks, command not found)
• Cross-platform path handling verification

🚀 Ready for Review

This implementation provides a robust, well-tested hook execution runtime that forms the core of the samoid Git hooks system. The dependency injection pattern ensures maintainability and testability while supporting all specified requirements.

Closes #3