Fix critical process daemonization issue by replacing std::mem::forget with proper background thread management

PROBLEM:
The previous implementation used std::mem::forget(child) to detach processes,
which caused several critical issues:
- Prevented Child destructor from running, leading to resource leaks
- Could result in zombie processes under certain conditions
- Violated Rust best practices for resource management
- No proper cleanup of OS handles and process resources

SOLUTION:
Replaced std::mem::forget(child) with a background thread approach:
- Spawn child process in background thread that owns the Child struct
- When thread completes, Child's Drop implementation runs automatically
- Ensures proper resource cleanup while maintaining process detachment
- Process becomes child of init (PID 1) which handles reaping
- Follows Rust idioms for resource management

VERIFICATION:
- Added test that verifies proper resource management
- All existing functionality preserved
- No breaking changes to CLI interface
- Improved system resource handling

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

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