fix: preserve default chat project fallback

packages/casan-harness/scripts/bash/chat-turn.py

@@ -703,7 +703,13 @@ def submit_escalation(args, router, binding):
 
 
 def ask(args) -> int:
-    os.environ["CASAN_PROJECT_ID"] = args.project or "default"
+    # "default" is the Control Panel's logical fallback, not a manifest project.
+    # Export only explicit project bindings so generic chat turns can use the
+    # repository domain fallback without weakening manifest validation.
+    if args.project and args.project != "default":
+        os.environ["CASAN_PROJECT_ID"] = args.project
+    else:
+        os.environ.pop("CASAN_PROJECT_ID", None)
     guard_rc = tenant_runtime_guard(args)
     if guard_rc != 0:
         return guard_rc