feat(backend): BACKEND_SLOT_PINNING — per-phase engine slot ownership

Each pipeline phase owns one llama-server slot (steward 0, orchestrator
1, synthesizer 2), carried as id_slot in extra_body through the same
mechanism tool_choice already uses, so the phase's stable prompt prefix
stays in that slot's KV cache and a turn re-prefills only its new
tokens. Off by default; a no-op on the Claude backend and ignored by
Ollama, so the flag is safe on any backend and the cutover itself stays
a pure env swap.

The merge helper preserves existing extra_body keys — mutation-checked
(dropping the merge fails exactly the test written for it).

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

src/agents/steward/agent.py

@@ -162,15 +162,20 @@ class StewardAgent:
         OpenAI-compatible server (Ollama, llama-server) can sit behind
         OLLAMA_HOST without this method knowing which.
         """
+        payload: dict = {
+            "model": self.ollama_model,
+            "messages": [{"role": "user", "content": prompt}],
+            "temperature": 0.3,  # Lower = more consistent
+            "top_p": 0.9,
+        }
+        if config.BACKEND_SLOT_PINNING:
+            # The steward owns engine slot 0 (see BACKEND_SLOT_PINNING)
+            payload["id_slot"] = 0
+
         async with httpx.AsyncClient(timeout=self.timeout) as client:
             response = await client.post(
                 f"{self.ollama_host}/v1/chat/completions",
-                json={
-                    "model": self.ollama_model,
-                    "messages": [{"role": "user", "content": prompt}],
-                    "temperature": 0.3,  # Lower = more consistent
-                    "top_p": 0.9,
-                },
+                json=payload,
             )
 
             response.raise_for_status()