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.

tests/agents/test_headless_runner_chunks.py

@@ -0,0 +1,68 @@
+"""``run_agent_headless`` passes the agent's ``chunks`` to the retriever.
+
+``chunks=0`` switches retrieval off, but ``int(... or 2)`` read it as unset,
+so a scheduled run of an agent with retrieval off retrieved anyway.
+"""
+
+from __future__ import annotations
+
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+
+def _retriever_chunks(agent_config, monkeypatch):
+    """The ``chunks`` a headless run hands to the retriever."""
+    from docsgpt.agents import headless_runner as hr
+
+    agent = MagicMock(name="agent")
+    agent.gen.return_value = iter([{"answer": "ok"}])
+    agent.llm.token_usage = {"prompt_tokens": 1, "generated_tokens": 1}
+
+    retriever = MagicMock(name="retriever")
+    retriever.search.return_value = []
+    created = {}
+
+    def create_retriever(cls, *args, **kwargs):
+        created.update(kwargs)
+        return retriever
+
+    tool_executor = MagicMock(name="tool_executor")
+    tool_executor.headless_denials = []
+
+    monkeypatch.setattr(hr, "get_prompt", lambda _pid: "system prompt")
+    monkeypatch.setattr(
+        hr.RetrieverCreator, "create_retriever", classmethod(create_retriever),
+    )
+    monkeypatch.setattr(hr, "ToolExecutor", lambda *a, **kw: tool_executor)
+    monkeypatch.setattr(
+        hr.AgentCreator, "create_agent",
+        classmethod(lambda cls, *a, **kw: agent),
+    )
+
+    config = {"user_id": "u1", "id": "agent-1", "default_model_id": "m", **agent_config}
+    with patch("docsgpt.core.model_utils.validate_model_id", return_value=True), \
+         patch("docsgpt.core.model_utils.get_default_model_id", return_value="m"), \
+         patch(
+             "docsgpt.core.model_utils.get_provider_from_model_id",
+             return_value="openai",
+         ), \
+         patch("docsgpt.core.model_utils.get_api_key_for_provider", return_value="k"), \
+         patch("docsgpt.utils.calculate_doc_token_budget", return_value=1000):
+        hr.run_agent_headless(config, "do the thing")
+    return created["chunks"]
+
+
+@pytest.mark.unit
+class TestHeadlessRunnerChunks:
+    def test_unset_chunks_uses_the_default(self, monkeypatch):
+        assert _retriever_chunks({}, monkeypatch) == 6
+
+    def test_null_chunks_uses_the_default(self, monkeypatch):
+        assert _retriever_chunks({"chunks": None}, monkeypatch) == 6
+
+    def test_zero_chunks_keeps_retrieval_off(self, monkeypatch):
+        assert _retriever_chunks({"chunks": 0}, monkeypatch) == 0
+
+    def test_explicit_chunks_is_kept(self, monkeypatch):
+        assert _retriever_chunks({"chunks": 4}, monkeypatch) == 4