Implement input auto-detection for string, file, directory, and stdin

[Matographo]

Summary

Implement positional-input resolution order and stdin fallback behavior.

Why

The product relies on smart CLI behavior for string, file, directory, and stdin workflows.

Deliverables

• File detection
• Directory detection
• Literal-string fallback
• stdin fallback when no positional input is provided

Acceptance Criteria

• Existing file selects file mode
• Existing directory selects directory mode
• Missing path becomes a literal string
• No positional input reads stdin
• Behavior is covered by tests

Dependencies

• Depends on Specify CLI behavior, inspection commands, and exit codes #2
• Depends on Create the base project structure for CLI, engine, parser, evaluator, and config #3