feat(backend): every generation call carries a session name

The release audit swept every agent.run and raw chat call: the two
streaming orchestrate paths never carried a binding (they predate the
v2.6.0 pinning), and the biographer and housekeeper ran bare. Through
the wrapper a session-less call takes idle slots only and 503s once
four sessions are resident — so the streaming paths join
tatlock-orchestrate, and the experts share tatlock-experts at rank 35:
between the librarian and the phases, the two least-used consumers
trade the spare slot by rank instead of anyone hitting an empty pool.
Rank mutation shown to fail its test.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/agents/housekeeper/agent.py

@@ -206,12 +206,12 @@ async def run_housekeeper(
 
     try:
         # Temperature 0.1 for slight exploration (skipped on Claude backend)
-        from src.anthropic.model_selector import get_sampling_settings
+        from src.anthropic.model_selector import get_sampling_settings, with_phase_binding
 
         result = await agent.run(
             prompt,
             message_history=message_history,
-            model_settings=get_sampling_settings(0.1),
+            model_settings=with_phase_binding(get_sampling_settings(0.1), "experts"),
         )
 
         logger.info(