Prevent agent mode from stopping after first TODO phase

- Add TODO completion check to final_output tool in autonomous mode only
- When incomplete TODO items exist, reject final_output and prompt LLM to continue
- Non-autonomous modes (interactive, chat) are unaffected
- Add 6 tests verifying behavior in both autonomous and non-autonomous modes

Fixes issue where LLM would call final_output after completing first phase,
causing agent to stop prematurely instead of continuing with remaining phases.

crates/g3-core/src/prompts.rs

@@ -71,7 +71,7 @@ Every multi-step task follows this pattern:
 1. **Start**: Call todo_read, then todo_write to create your plan
 2. **During**: Execute steps, then todo_read and todo_write to mark progress
 3. **End**: Call todo_read to verify all items complete
-    
+
 Note: todo_write replaces the entire todo.g3.md file, so always read first to preserve content. TODO lists are scoped to the current session and stored in the session directory.
 
 IMPORTANT: If you are provided with a SHA256 hash of the requirements file, you MUST include it as the very first line of the todo.g3.md file in the following format: