Default retrieval to 6 chunks instead of 2

chunks is a total per request, split across the attached sources, so an
agent with two sources and the default of 2 got a single chunk from each,
and its answers changed with whichever chunk won. 6 gives three per
source for about 4-5k more input tokens per retrieval turn.

Every literal default moves from 2 to 6: the request default, the
retrievers, the internal search tool, workflow agent nodes, scheduled and
headless runs, agent create/update/import, the source retrieval config and
the frontend forms. Existing agents and sources keep what they store; a
source saved with chunks=2 now counts as configured at 2, which is pinned
by a test.

Headless runs also read chunks=0 as unset (`or 2`), so an agent with
retrieval switched off retrieved anyway on scheduled runs; 0 now stays 0.

docsgpt/api/user/scheduler_worker.py

@@ -74,7 +74,7 @@ def _ephemeral_agent_for_agentless(
         "user_id": user_id,
         "agent_type": "classic",
         "retriever": "classic",
-        "chunks": 2,
+        "chunks": 6,
         "prompt_id": "default",
         "source_id": None,
         "default_model_id": schedule.get("model_id") or "",