- Replaced go-gitignore with custom GitignoreStack that tracks
  multiple .gitignore files hierarchically
- Patterns are processed in root-to-leaf order, last match wins
- Negation patterns in child dirs override parent ignores
- Patterns only apply within their directory scope (no leaking)
- Added 6 new tests for nested .gitignore:
  - Scoped: patterns only affect own directory
  - Dir-scoped: different dirs can ignore different things
  - Negation override: child !pattern un-ignores parent's *.pattern
  - Deep nesting: 3+ levels of .gitignore
  - Parent scope doesn't leak to siblings
- All 35 tests passing

- Added gitignore_test.go: comprehensive tests for go-gitignore patterns
  - Verified: *, **, [], negation, anchored patterns, comments
  - Documented limitation: ? wildcard not supported by go-gitignore
- Added sync_test.go: integration tests for initial sync
  - Tests: basic files, gitignore excludes, negation, double-star,
    directory patterns, symlinks, permissions, dry-run, .git skip,
    nested gitignore, complex combined patterns
- Fixed gitignore.go: use CompileIgnoreFile instead of passing
  entire file content as single string to CompileIgnoreLines