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/tatlock.py

@@ -536,13 +536,13 @@ class TatlockAgent(AgentInterface):
 
         # Run with scoped tools and tracker
         # Force tool_choice to make LLM actually call tools
-        from src.anthropic.model_selector import get_tool_choice_settings
+        from src.anthropic.model_selector import get_tool_choice_settings, with_slot_pinning
 
         result = await scoped_agent.run(
             enriched_message,
             message_history=pydantic_history if pydantic_history else None,
             deps=tool_tracker,
-            model_settings=get_tool_choice_settings(),
+            model_settings=with_slot_pinning(get_tool_choice_settings(), slot=1),
         )
 
         logger.info(
@@ -725,13 +725,13 @@ class TatlockAgent(AgentInterface):
                 pydantic_history.append(ModelResponse(parts=[TextPart(content=content)]))
 
         # Run with scoped tools and tracker
-        from src.anthropic.model_selector import get_tool_choice_settings
+        from src.anthropic.model_selector import get_tool_choice_settings, with_slot_pinning
 
         result = await scoped_agent.run(
             enriched_message,
             message_history=pydantic_history if pydantic_history else None,
             deps=tool_tracker,
-            model_settings=get_tool_choice_settings(),
+            model_settings=with_slot_pinning(get_tool_choice_settings(), slot=1),
         )
 
         # Extract tool calls and results from the agent's messages
@@ -888,9 +888,12 @@ class TatlockAgent(AgentInterface):
                 pydantic_history.append(ModelResponse(parts=[TextPart(content=content)]))
 
         # Run synthesis
+        from src.anthropic.model_selector import with_slot_pinning
+
         result = await synthesis_agent.run(
             synthesis_prompt,
             message_history=pydantic_history if pydantic_history else None,
+            model_settings=with_slot_pinning(None, slot=2),
         )
 
         logger.info(