fix: prevent testing agents from running indefinitely

This fix addresses two root causes that caused testing agents to
accumulate (10-12 agents) instead of maintaining a 1:1 ratio with
coding agents:

1. Testing agents now exit after one session (agent.py)
   - Added `or agent_type == "testing"` to the exit condition
   - Previously, testing agents never hit the exit condition since
     they're spawned with feature_id=None

2. Testing agents now spawn when coding agents START, not complete
   - Moved spawn logic from _on_agent_complete() to start_feature()
   - Removed the old spawn logic from _on_agent_complete()
   - This ensures proper 1:1 ratio and prevents accumulation

Expected behavior after fix:
- First coding agent: no testing agent (no passing features yet)
- Subsequent coding agents: one testing agent spawns per start
- Each testing agent tests ONE feature then terminates immediately

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

agent.py

@@ -306,9 +306,12 @@ async def run_autonomous_agent(
                 print("The autonomous agent has finished its work.")
                 break
 
-            # Single-feature mode: exit after one session (orchestrator manages agents)
-            if feature_id is not None:
-                print(f"\nSingle-feature mode: Feature #{feature_id} session complete.")
+            # Single-feature mode OR testing agent: exit after one session
+            if feature_id is not None or agent_type == "testing":
+                if agent_type == "testing":
+                    print("\nTesting agent complete. Terminating session.")
+                else:
+                    print(f"\nSingle-feature mode: Feature #{feature_id} session complete.")
                 break
 
             await asyncio.sleep(delay_seconds)