refactor: remove AX_DB_PATH env var support from store layer

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

src/store/graph_store_sqlite.go

@@ -77,12 +77,6 @@ func FindAndOpenSQLiteStore() (GraphStore, error) {
 // mode: if no .ax.db is found it creates and initialises one in the current
 // working directory instead of returning an error.
 func FindOrInitSQLiteStore() (GraphStore, error) {
-	if dbpath := os.Getenv("AX_DB_PATH"); dbpath != "" {
-		if err := InitSQLiteStore(dbpath); err != nil {
-			return nil, err
-		}
-		return NewSQLiteStore(dbpath)
-	}
 	dir, err := filepath.Abs(".")
 	if err != nil {
 		return nil, err