feat: add optional model override for spawn subagents

Threads an optional 'model' parameter through SpawnTool → SubagentManager.spawn() → _run_subagent() → provider.chat(). Falls back to self.model when not specified.

Usage: spawn(task='...', model='claude-sonnet-4-20250514')

nanobot/agent/subagent.py

@@ -50,6 +50,7 @@ class SubagentManager:
         self,
         task: str,
         label: str | None = None,
+        model: str | None = None,
         origin_channel: str = "cli",
         origin_chat_id: str = "direct",
     ) -> str:
@@ -75,7 +76,7 @@ class SubagentManager:
         
         # Create background task
         bg_task = asyncio.create_task(
-            self._run_subagent(task_id, task, display_label, origin)
+            self._run_subagent(task_id, task, display_label, origin, model=model)
         )
         self._running_tasks[task_id] = bg_task
         
@@ -91,6 +92,7 @@ class SubagentManager:
         task: str,
         label: str,
         origin: dict[str, str],
+        model: str | None = None,
     ) -> None:
         """Execute the subagent task and announce the result."""
         logger.info(f"Subagent [{task_id}] starting task: {label}")
@@ -129,7 +131,7 @@ class SubagentManager:
                 response = await self.provider.chat(
                     messages=messages,
                     tools=tools.get_definitions(),
-                    model=self.model,
+                    model=model or self.model,
                 )
                 
                 if response.has_tool_calls:

nanobot/agent/tools/spawn.py

@@ -51,15 +51,20 @@ class SpawnTool(Tool):
                     "type": "string",
                     "description": "Optional short label for the task (for display)",
                 },
+                "model": {
+                    "type": "string",
+                    "description": "Optional model override for the subagent (e.g. 'claude-sonnet-4-20250514'). Defaults to the main agent's model.",
+                },
             },
             "required": ["task"],
         }
     
-    async def execute(self, task: str, label: str | None = None, **kwargs: Any) -> str:
+    async def execute(self, task: str, label: str | None = None, model: str | None = None, **kwargs: Any) -> str:
         """Spawn a subagent to execute the given task."""
         return await self._manager.spawn(
             task=task,
             label=label,
+            model=model,
             origin_channel=self._origin_channel,
             origin_chat_id=self._origin_chat_id,
         )