Handle negation patterns (!pattern)

### Summary
Add support for negation patterns (`!pattern`) and overlapping ignore rules.

### Acceptance Criteria
- Patterns like `!keep.ts` correctly un-ignore files
- Verbose mode shows which rule overrode another

Start by locating the ignore-rule parsing and matching entry points, then trace how verbose mode reports rule decisions. Add coverage for !pattern un-ignore behavior, overlapping rules, and the override message. Done means !keep.ts is restored correctly and verbose output identifies which rule took precedence.

Written by the indexing model from the issue text.