Search every agent source in scheduled and webhook runs

Headless runs searched only the agent's primary source, so an agent whose
knowledge sat in its extra sources answered a schedule or webhook without
it. They now take the primary and every extra source through the same
owner-or-sponsor check a chat uses, shared as one helper, and retrieve
through the per-source dispatcher so each source keeps its own settings.

tests/worker/test_agent_workers.py

@@ -375,4 +375,4 @@ class TestRunAgentHeadlessFromWebhook:
         outcome = headless_runner.run_agent_headless(agent_config, "hello")
 
         assert outcome["answer"] == "done"
-        assert captured_source.get("active_docs") == source_id
+        assert captured_source.get("active_docs") == [source_id]