feat: reduce default node ID length to 3 and auto-grow when exhausted

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/store/graph_store_sqlite.go

@@ -265,8 +265,8 @@ func (s *txStore) NodeExists(id string) (bool, error) { return nodeExists(s.db,
 
 // --- ID generation ---
 
-func genID() string {
-	b := make([]byte, 5)
+func genID(length int) string {
+	b := make([]byte, length)
 	for i := range b {
 		b[i] = "abcdefghijklmnopqrstuvwxyz"[rand.Intn(26)]
 	}
@@ -274,8 +274,14 @@ func genID() string {
 }
 
 func generateID(q querier) (string, error) {
-	for {
-		id := genID()
+	for length := 3; ; length++ {
+		// Try up to 10*capacity attempts at this length before growing.
+		capacity := 1
+		for i := 0; i < length; i++ {
+			capacity *= 26
+		}
+		for attempt := 0; attempt < 10*capacity; attempt++ {
+			id := genID(length)
 			exists, err := nodeExists(q, id)
 			if err != nil {
 				return "", err
@@ -284,6 +290,7 @@ func generateID(q querier) (string, error) {
 				return id, nil
 			}
 		}
+	}
 }
 
 func (s *GraphStoreSqlite) GenerateID() (string, error) { return generateID(s.db) }