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/anthropic/model_selector.py

@@ -313,6 +313,12 @@ _PHASE_BINDINGS: dict[str, tuple[int | None, str, int]] = {
     "steward": (0, "tatlock-steward", 40),
     "orchestrate": (1, "tatlock-orchestrate", 40),
     "synthesize": (2, "tatlock-synthesize", 40),
+    # Experts (biographer, housekeeper) share one session between the
+    # librarian and the phases: the two least-used consumers trade the
+    # spare slot by rank instead of anyone hitting an empty pool — a
+    # session-less call 503s once four sessions are resident (D-4's
+    # idle-only rule), so every generation call here must carry a name.
+    "experts": (None, "tatlock-experts", 35),
     "librarian": (None, "librarian", 30),
 }