feat: skip already-indexed revisions, add --force flag

When indexing a revision that already exists, the indexer now returns
early with information about the existing revision instead of re-indexing.
Use the --force flag to re-index an existing revision.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/nixos/indexer.go

@@ -35,10 +35,11 @@ func NewIndexer(store database.Store) *Indexer {
 
 // IndexResult contains the results of an indexing operation.
 type IndexResult struct {
-	Revision    *database.Revision
-	OptionCount int
-	FileCount   int
-	Duration    time.Duration
+	Revision       *database.Revision
+	OptionCount    int
+	FileCount      int
+	Duration       time.Duration
+	AlreadyIndexed bool // True if revision was already indexed (skipped)
 }
 
 // IndexRevision indexes a nixpkgs revision by git hash or channel name.
@@ -55,9 +56,10 @@ func (idx *Indexer) IndexRevision(ctx context.Context, revision string) (*IndexR
 	}
 	if existing != nil {
 		return &IndexResult{
-			Revision:    existing,
-			OptionCount: existing.OptionCount,
-			Duration:    time.Since(start),
+			Revision:       existing,
+			OptionCount:    existing.OptionCount,
+			Duration:       time.Since(start),
+			AlreadyIndexed: true,
 		}, nil
 	}
 
@@ -112,6 +114,25 @@ func (idx *Indexer) IndexRevision(ctx context.Context, revision string) (*IndexR
 	}, nil
 }
 
+// ReindexRevision forces re-indexing of a revision, deleting existing data first.
+func (idx *Indexer) ReindexRevision(ctx context.Context, revision string) (*IndexResult, error) {
+	ref := resolveRevision(revision)
+
+	// Delete existing revision if present
+	existing, err := idx.store.GetRevision(ctx, ref)
+	if err != nil {
+		return nil, fmt.Errorf("failed to check existing revision: %w", err)
+	}
+	if existing != nil {
+		if err := idx.store.DeleteRevision(ctx, existing.ID); err != nil {
+			return nil, fmt.Errorf("failed to delete existing revision: %w", err)
+		}
+	}
+
+	// Now index fresh
+	return idx.IndexRevision(ctx, revision)
+}
+
 // buildOptions builds options.json for a nixpkgs revision.
 func (idx *Indexer) buildOptions(ctx context.Context, ref string) (string, func(), error) {
 	// Create temp directory