Implement nested .gitignore support with GitignoreStack

- 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

1 files changed, 3 insertions, 6 deletions

diff --git a/main.go b/main.go
index 436c7b7..24135f5 100644
--- a/main.go
+++ b/main.go
@@ -25,12 +25,9 @@ func main() {
 		log.Printf("host: %s, data: %s, dry-run: %v", *hostDir, *dataDir, *dryRun)
 	}
 
-	rules, err := loadGitignore(*hostDir)
-	if err != nil {
-		log.Fatalf("failed to load .gitignore: %v", err)
-	}
+	stack := NewGitignoreStack(*hostDir)
 
-	manifest, err := initialSync(*hostDir, *dataDir, rules, *dryRun)
+	manifest, err := initialSync(*hostDir, *dataDir, stack, *dryRun)
 	if err != nil {
 		log.Fatalf("initial sync failed: %v", err)
 	}
@@ -38,7 +35,7 @@ func main() {
 	ctx, cancel := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
 	defer cancel()
 
-	if err := watch(ctx, *hostDir, *dataDir, manifest, rules, *dryRun, *verbose); err != nil {
+	if err := watch(ctx, *hostDir, *dataDir, manifest, stack, *dryRun, *verbose); err != nil {
 		log.Fatalf("watcher failed: %v", err)
 	}
 }