fix: create output directory if it doesn't exist

[harp-intel]

Summary

Fixes #556 by automatically creating the output directory when using the --output flag, rather than requiring it to exist beforehand.

Problem

Previously, when using --output /path/to/dir, PerfSpect would exit with an error if the directory didn't exist:

Error: requested output dir, /path/to/dir, does not exist

This was inconsistent with the default behavior where output directories are created automatically.

Solution

Replace the existence check with a call to common.CreateOutputDir(), which:

• Creates the directory if it doesn't exist using os.MkdirAll
• Creates parent directories as needed (e.g., --output /path/to/nested/dir)
• Sets appropriate permissions (0755)
• Safely handles the case where the directory already exists
• Provides clear error messages if creation fails (e.g., permission denied)

Changes

File: cmd/root.go

Replaced lines 160-168:

exists, err := util.DirectoryExists(outputDir)
if err != nil {
    fmt.Printf("Error: failed to determine if output dir exists: %v\n", err)
    os.Exit(1)
}
if !exists {
    fmt.Printf("Error: requested output dir, %s, does not exist\n", outputDir)
    os.Exit(1)
}

With:

// Create the directory if it doesn't exist
if err := common.CreateOutputDir(outputDir); err != nil {
    fmt.Printf("Error: failed to create output directory: %v\n", err)
    os.Exit(1)
}

Benefits

1. Improved usability - Users no longer need to manually create directories
2. Consistent behavior - --output now works like default output directories
3. Safe implementation - Uses existing, tested infrastructure
4. Better user experience - Creates nested directories automatically

Testing

✅ All existing unit tests pass
✅ Code quality checks pass (make format, make test)

Manual testing scenarios to verify:

• --output /tmp/newdir creates the directory
• --output /existing/dir works without error
• --output /deeply/nested/new/dir creates all parents
• Invalid paths (e.g., no permissions) fail with clear error message