feat: add install-to subcommand for binary self-installation

- Add new 'install-to DIR' subcommand that copies the running binary to a specified directory
- Uses atomic operations: copy to temp file, then rename for atomicity
- Preserves executable permissions (755) from original binary
- Handles errors gracefully with proper exit codes and cleanup
- Validates destination directory exists and is actually a directory
- Integrated with existing help system and command structure

Implementation details:
- New file: internal/cli/commands_install_to.go - core functionality
- New file: internal/cli/commands_install_to_test.go - comprehensive test suite
- Modified: internal/cli/main.go - CLI integration and command parsing

Testing:
- 7 test functions covering unit and integration testing
- Tests atomic operations, error handling, permission preservation
- Tests CLI interface, help system, and edge cases
- All tests passing

This feature enables easy binary deployment and self-updating workflows
while maintaining compatibility with upstream migrate project structure.

Author: daniel-garcia

internal/cli/commands_install_to.go

@@ -0,0 +1,81 @@
+package cli
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
+)
+
+func installToCmd(destDir string) error {
+	// Get the path to the current executable
+	executablePath, err := os.Executable()
+	if err != nil {
+		return fmt.Errorf("failed to get executable path: %w", err)
+	}
+
+	// Get the base name of the executable
+	executableName := filepath.Base(executablePath)
+	
+	// Create destination path
+	destPath := filepath.Join(destDir, executableName)
+	tempPath := destPath + ".tmp"
+
+	// Remove temp file on error
+	defer func() {
+		if err != nil {
+			if _, statErr := os.Stat(tempPath); statErr == nil {
+				os.Remove(tempPath)
+			}
+		}
+	}()
+
+	// Get source file info to preserve permissions
+	sourceInfo, err := os.Stat(executablePath)
+	if err != nil {
+		return fmt.Errorf("failed to get source file info: %w", err)
+	}
+
+	// Open source file
+	sourceFile, err := os.Open(executablePath)
+	if err != nil {
+		return fmt.Errorf("failed to open source file: %w", err)
+	}
+	defer sourceFile.Close()
+
+	// Create temp destination file
+	tempFile, err := os.OpenFile(tempPath, os.O_CREATE|os.O_WRONLY|os.O_TRUNC, 0755)
+	if err != nil {
+		return fmt.Errorf("failed to create temp file: %w", err)
+	}
+	defer tempFile.Close()
+
+	// Copy the file content
+	_, err = io.Copy(tempFile, sourceFile)
+	if err != nil {
+		return fmt.Errorf("failed to copy file content: %w", err)
+	}
+
+	// Ensure all writes are flushed
+	err = tempFile.Sync()
+	if err != nil {
+		return fmt.Errorf("failed to sync temp file: %w", err)
+	}
+
+	// Close the temp file before renaming
+	tempFile.Close()
+
+	// Set the correct permissions (preserve executable bit)
+	err = os.Chmod(tempPath, sourceInfo.Mode())
+	if err != nil {
+		return fmt.Errorf("failed to set permissions: %w", err)
+	}
+
+	// Atomically move temp file to final destination
+	err = os.Rename(tempPath, destPath)
+	if err != nil {
+		return fmt.Errorf("failed to move temp file to destination: %w", err)
+	}
+
+	return nil
+}