refactor(agent): simplify subagent concurrency with rejection over semaphore

Replace the asyncio.Semaphore queueing approach with a simple count
check in SpawnTool.execute(). When the concurrency limit is reached,
the tool returns an error string so the agent can perceive the reason
and adjust its behavior instead of silently queueing.

- Remove max_concurrent_subagents parameter threading through
  AgentLoop, commands.py, and nanobot.py
- SubagentManager reads the limit directly from AgentDefaults
- SpawnTool checks get_running_count() before calling spawn()
- Simplify tests to verify rejection behavior

nanobot/agent/tools/spawn.py

@@ -56,6 +56,14 @@ class SpawnTool(Tool):
 
     async def execute(self, task: str, label: str | None = None, **kwargs: Any) -> str:
         """Spawn a subagent to execute the given task."""
+        running = self._manager.get_running_count()
+        limit = self._manager.max_concurrent_subagents
+        if running >= limit:
+            return (
+                f"Cannot spawn subagent: concurrency limit reached "
+                f"({running}/{limit} running). Wait for a running subagent "
+                f"to complete before spawning a new one."
+            )
         return await self._manager.spawn(
             task=task,
             label=label,