feat(index-builder): generate data/meta/index.json

Adds MetaIndex struct and generateMetaIndex function to create
data/meta/index.json listing all available meta data files
(archetypes.json, rivalries.json) with descriptions.

Also adds the new file to the R2 warm cache upload list.

Closes: bf-66rk

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

cmd/acb-index-builder/generator.go

@@ -175,6 +175,11 @@ func generateAllIndexes(data *IndexData, outputDir string, db *sql.DB, cfg *Conf
 		return fmt.Errorf("archetypes: %w", err)
 	}
 
+	// Generate meta directory index (data/meta/index.json)
+	if err := generateMetaIndex(outputDir); err != nil {
+		return fmt.Errorf("meta index: %w", err)
+	}
+
 	// Generate community hints (data/evolution/community_hints.json)
 	if err := generateCommunityHints(data, outputDir); err != nil {
 		return fmt.Errorf("community hints: %w", err)
@@ -1898,6 +1903,18 @@ type ArchetypesIndex struct {
 	Archetypes []ArchetypeEntry `json:"archetypes"`
 }
 
+// MetaFileEntry represents a single meta data file in the meta directory index.
+type MetaFileEntry struct {
+	Name        string `json:"name"`        // e.g., "archetypes.json"
+	Description string `json:"description"` // e.g., "Bot archetype classifications"
+}
+
+// MetaIndex is the top-level structure for data/meta/index.json.
+type MetaIndex struct {
+	UpdatedAt string          `json:"updated_at"`
+	Files     []MetaFileEntry `json:"files"`
+}
+
 // classifyArchetype infers a behavioral archetype from bot name when the
 // archetype field is empty.
 func classifyArchetype(bot BotData) string {
@@ -1983,6 +2000,26 @@ func generateArchetypes(data *IndexData, outputDir string) error {
 	return writeJSON(filepath.Join(metaDir, "archetypes.json"), index)
 }
 
+// generateMetaIndex builds data/meta/index.json listing all available meta data files.
+func generateMetaIndex(outputDir string) error {
+	metaDir := filepath.Join(outputDir, "data", "meta")
+	if err := os.MkdirAll(metaDir, 0755); err != nil {
+		return err
+	}
+
+	// List all available meta data files with descriptions
+	files := []MetaFileEntry{
+		{Name: "archetypes.json", Description: "Bot archetype classifications and statistics"},
+		{Name: "rivalries.json", Description: "Top rivalries between bots"},
+	}
+
+	index := MetaIndex{
+		UpdatedAt: time.Now().Format(time.RFC3339),
+		Files:     files,
+	}
+	return writeJSON(filepath.Join(metaDir, "index.json"), index)
+}
+
 // ─── Community Hints (§15.2 / §13.6) ──────────────────────────────────────────
 
 // CommunityHint is a single high-upvote tactical insight from §13.6 feedback.

cmd/acb-index-builder/main.go

@@ -124,6 +124,7 @@ func uploadMetaJSONToR2(ctx context.Context, cfg *Config, outputDir string, data
 	}
 
 	static := []string{
+		"data/meta/index.json",
 		"data/meta/archetypes.json",
 		"data/meta/rivalries.json",
 		"data/evolution/community_hints.json",

cmd/acb-index-builder/main_test.go

@@ -1546,3 +1546,52 @@ func TestGenerateMapsIndex(t *testing.T) {
 		t.Errorf("Expected 0 cores for map_def456, got %d", len(detail2.Cores))
 	}
 }
+
+func TestGenerateMetaIndex(t *testing.T) {
+	tmpDir := t.TempDir()
+
+	if err := generateMetaIndex(tmpDir); err != nil {
+		t.Fatalf("generateMetaIndex failed: %v", err)
+	}
+
+	content, err := os.ReadFile(filepath.Join(tmpDir, "data", "meta", "index.json"))
+	if err != nil {
+		t.Fatalf("Failed to read meta/index.json: %v", err)
+	}
+
+	var index MetaIndex
+	if err := json.Unmarshal(content, &index); err != nil {
+		t.Fatalf("Failed to parse meta/index.json: %v", err)
+	}
+
+	if len(index.Files) != 2 {
+		t.Errorf("Expected 2 files in meta index, got %d", len(index.Files))
+	}
+	if index.UpdatedAt == "" {
+		t.Error("UpdatedAt should not be empty")
+	}
+
+	// Verify archetypes.json entry
+	foundArchetypes := false
+	foundRivalries := false
+	for _, f := range index.Files {
+		if f.Name == "archetypes.json" {
+			foundArchetypes = true
+			if f.Description == "" {
+				t.Error("archetypes.json should have a description")
+			}
+		}
+		if f.Name == "rivalries.json" {
+			foundRivalries = true
+			if f.Description == "" {
+				t.Error("rivalries.json should have a description")
+			}
+		}
+	}
+	if !foundArchetypes {
+		t.Error("Expected archetypes.json entry in meta index")
+	}
+	if !foundRivalries {
+		t.Error("Expected rivalries.json entry in meta index")
+	}
+}