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/mcp/handlers.go

@@ -217,6 +217,20 @@ func (s *Server) makeIndexHandler(indexer *nixos.Indexer) ToolHandler {
 			return ErrorContent(fmt.Errorf("indexing failed: %w", err)), nil
 		}
 
+		// If already indexed, return early with info
+		if result.AlreadyIndexed {
+			var sb strings.Builder
+			sb.WriteString(fmt.Sprintf("Revision already indexed: %s\n", result.Revision.GitHash))
+			if result.Revision.ChannelName != "" {
+				sb.WriteString(fmt.Sprintf("Channel: %s\n", result.Revision.ChannelName))
+			}
+			sb.WriteString(fmt.Sprintf("Options: %d\n", result.OptionCount))
+			sb.WriteString(fmt.Sprintf("Indexed at: %s\n", result.Revision.IndexedAt.Format("2006-01-02 15:04")))
+			return CallToolResult{
+				Content: []Content{TextContent(sb.String())},
+			}, nil
+		}
+
 		// Index files by default
 		fileCount, err := indexer.IndexFiles(ctx, result.Revision.ID, result.Revision.GitHash)
 		if err != nil {