Remove final_output tool - let summaries stream naturally

- Remove final_output from tool definitions, dispatch, and misc tools
- Update system prompts to request summaries as regular markdown text
- Remove print_final_output from UiWriter trait and all implementations
- Remove final_output handling from agent core logic
- Rename final_output_summary → summary in session continuation
- Delete final_output test files
- Update tool count tests (12→11, 27→26)

This allows LLM summaries to stream through the markdown formatter
for a more natural, responsive user experience instead of buffering
everything into a tool call.

crates/g3-core/tests/todo_staleness_test.rs

@@ -81,9 +81,6 @@ impl UiWriter for MockUiWriter {
             .push(format!("CHOICE: {} Options: {:?}", message, options));
         self.choice_responses.lock().unwrap().pop().unwrap_or(0)
     }
-    fn print_final_output(&self, summary: &str) {
-        self.output.lock().unwrap().push(format!("FINAL: {}", summary));
-    }
 }
 
 #[tokio::test]