fix: use dedicated tool-phase prompt for gemma4 orchestration

With the butler persona prompt attached, gemma4 reasons about calling the
calculator and then answers from memory with a different wrong product every
run; tool_choice=required via extra_body is advisory at best on Ollama's
OpenAI-compat layer. orchestrate_tool_calls() now uses a terse
TATLOCK_ORCHESTRATION_PROMPT; synthesize_from_results() keeps the persona,
so the user-visible voice is unchanged.

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

src/agents/tatlock.py

@@ -129,6 +129,22 @@ or
 """
 
 
+# Tool-phase prompt for orchestrate_tool_calls(). The butler personality prompt
+# suppresses tool calling on small local models (gemma4 reasons about the tool,
+# then answers from memory with wrong arithmetic), so the orchestration phase
+# uses a terse operator prompt; synthesize_from_results() applies the persona.
+TATLOCK_ORCHESTRATION_PROMPT = """You are the tool-execution phase of Tatlock, \
+a butler assistant. Your only job is to gather accurate results by calling the \
+provided tools.
+
+- ALWAYS use tools for the task - never answer from memory and never do mental math.
+- Mathematics: call the calculate tool, even for trivial arithmetic.
+- Dates and times: call the date/time tools, never guess.
+- When the instructions say DELEGATE to an agent, call the matching delegate_to_* tool.
+- After the tool results arrive, reply with a one-line factual summary of the results. \
+A later step writes the polished reply, so do not add personality."""
+
+
 class TatlockAgent(AgentInterface):
     """
     Tatlock - The Butler agent using PydanticAI with Ollama.
@@ -654,10 +670,10 @@ class TatlockAgent(AgentInterface):
         # Create a fresh agent instance with scoped tools only
         model = get_model()
 
-        # Create agent with scoped tools
+        # Create agent with scoped tools, using the tool-phase prompt
         scoped_agent = Agent(
             model,
-            system_prompt=TATLOCK_SYSTEM_PROMPT,
+            system_prompt=TATLOCK_ORCHESTRATION_PROMPT,
             tools=scoped_tools,
         )