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>

tests/anthropic/test_model_selector.py

@@ -121,11 +121,12 @@ class TestPhaseBinding:
         monkeypatch.setattr(model_selector, "_local_flavor", "boilerroom")
         ranks = {
             phase: model_selector.phase_extra_body(phase)["eviction_order"]
-            for phase in ("steward", "orchestrate", "synthesize", "librarian")
+            for phase in ("steward", "orchestrate", "synthesize", "experts", "librarian")
         }
         assert ranks["steward"] == ranks["orchestrate"] == ranks["synthesize"] == 40
+        assert ranks["experts"] == 35
         assert ranks["librarian"] == 30
-        assert ranks["librarian"] < ranks["steward"]
+        assert ranks["librarian"] < ranks["experts"] < ranks["steward"]
         assert ranks["librarian"] > 20  # webber's rank stays below
 
     def test_wrapper_session_names_are_stable(self, local_first, monkeypatch):
@@ -134,12 +135,13 @@ class TestPhaseBinding:
         monkeypatch.setattr(model_selector, "_local_flavor", "boilerroom")
         names = {
             phase: model_selector.phase_extra_body(phase)["session"]
-            for phase in ("steward", "orchestrate", "synthesize", "librarian")
+            for phase in ("steward", "orchestrate", "synthesize", "experts", "librarian")
         }
         assert names == {
             "steward": "tatlock-steward",
             "orchestrate": "tatlock-orchestrate",
             "synthesize": "tatlock-synthesize",
+            "experts": "tatlock-experts",
             "librarian": "librarian",
         }