Adds the initial check command with console output.

Scope:

• add check subcommand;
• discover supported instruction files from a repository root;
• print human-readable console output;
• return 0 when supported files are found;
• return 1 when no supported files are found;
• return 2 for invalid repository roots;
• add CLI tests for the new behavior.

Not included:

• no JSON output;
• no Markdown output;
• no diagnostic finding generation;
• no file content analysis;
• no command execution;
• no network behavior;
• no CI changes.