fix(contextual): inject date_context + language in run_contextual_stream

Use _build_system_prompt helper so the contextual agent gets the
same system-prompt slots as home/floating runners — most importantly
{date_context} so the agent can reason about due dates when
creating/updating tasks.

Also makes the session_id contract on run_contextual_stream explicit
(was reading via context['_debug']) and tightens the tool-list test.

tests/test_run_contextual.py

@@ -70,5 +70,7 @@ async def test_run_contextual_stream_includes_scope_block(monkeypatch):
         "at least one entity-create tool must be present"
     )
 
+    assert "create_timeline" in names, "create_timeline tool must be included"
+
     # Note edit tools must NOT be exposed.
     assert "propose_note_edit" not in names, "propose_note_edit must be excluded"