feat(agents): make sources optional and drop the synthetic "Default" source

The sources list used to start with a fake "Default" entry that had no id
and, at run time, meant "no source, skip retrieval". The agent form
pre-selected it, snapped back to it when the last source was deselected,
and refused to publish without it, so a new agent always looked like it
had a knowledge base when it had none.

Backend
- /api/sources returns only ingested sources; no placeholder row.
- Publishing an agent no longer requires a source on create or update.
  The legacy "default" value is still accepted and maps to NULL.

Frontend
- The agent source picker starts empty, can be cleared, and shows a hint
  that a source-less agent answers from the model and its tools only.
- The picker groups sources into "Your sources" and "Shared with team"
  when any team-shared source exists, shows "N sources selected" for a
  multi-selection, and gets the same "Go to Sources" / "Upload new"
  footer as the chat picker. A source uploaded from the form is selected
  when it lands.
- Source selection serialisation and the picker id live in one helper
  shared with the chat picker; the four copies in the form are gone.
- The client no longer seeds a placeholder source in the store, and the
  dead auto-select of a "default" document is removed.

docsgpt/api/user/sources/routes.py

@@ -73,16 +73,10 @@ class CombinedJson(Resource):
         if not decoded_token:
             return make_response(jsonify({"success": False}), 401)
         user = decoded_token.get("sub")
-        data = [
-            {
-                "name": "Default",
-                "date": "default",
-                "model": settings.EMBEDDINGS_NAME,
-                "location": "remote",
-                "tokens": "",
-                "retriever": "classic",
-            }
-        ]
+        # Only ingested sources are listed. There is no synthetic "Default"
+        # entry: "no source" is expressed by an empty selection on the client,
+        # and the answer path treats a missing source as "skip retrieval".
+        data = []
 
         try:
             with db_readonly() as conn: