`clash check` - rich: include summary of what changed

[mathew55]

Extend clash check output to include a human/agent-readable summary on top of the raw diff.

Examples:

• "Modified authentication logic in lines 45-89"
• "Added new function validate_token()"
• "Deleted error handling block"

This could be:

• Heuristic-based - parse diff hunks, detect added/removed functions
• LLM-based - send diff to a model, get summary back

Depends on: #3. May or may not be worth building - see how useful the raw diff is first.