fix: expand trailing slash ignore patterns

[snvtac]

Fixes #579 by treating plain directory entries (for example vendor/) in .code-review-graphignore as recursive ignore rules.

What's changed

• Normalize ignore file entries in _load_ignore_patterns:
  • Strip a leading / for compatibility
  • Expand trailing / to /** so directory-only entries are respected
• Add regression coverage for vendor/ and generated/ in tests/test_incremental.py.

Validation

• Added unit test coverage in tests/test_incremental.py.

[itxaiohanglover]

Nice fix! Treating trailing-slash patterns as recursive globs matches .gitignore semantics. The leading slash strip is a good touch too. Test with vendor/ → vendor/** is exactly the right regression case.