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/answer/services/stream_processor.py

@@ -647,9 +647,9 @@ class StreamProcessor:
     def _configure_source(self):
         """Configure the source based on agent data.
 
-        The literal string ``"default"`` is a placeholder meaning "no
-        ingested source" and is normalized to an empty source so that no
-        retrieval is attempted.
+        The literal string ``"default"`` is a legacy placeholder (older
+        clients sent it for "no ingested source") and is normalized to an
+        empty source so that no retrieval is attempted.
         """
         if self._agent_data:
             agent_data = self._agent_data