feat: D24 fact intervals (--as-of) and bin/stack assistant helpers. (#37)

Store valid_from/valid_to on leafs and filter search by calendar day without
overloading D16 source staleness; stack start/start-assistant wires brain + PicoClaw.

internal/brain/db.go

@@ -85,9 +85,18 @@ func openWithSandbox(epsv string) error {
 		closeBrain()
 		return fmt.Errorf("LOAD EXTENSION VECTOR: %w", err)
 	}
+	migrateIntervalColumns()
 	return nil
 }
 
+// migrateIntervalColumns adds D24 valid_from/valid_to on existing Leaf tables.
+// Fresh CREATE already has them; ALTER is a no-op when the column exists.
+func migrateIntervalColumns() {
+	for _, col := range []string{"valid_from", "valid_to"} {
+		_, _ = conn.Query("ALTER TABLE Leaf ADD " + col + " STRING")
+	}
+}
+
 func closeBrain() {
 	if conn != nil {
 		conn.Close()