Remove final_output tool and improve scout report handback

final_output removal:
- Remove final_output from tool definitions and dispatch
- Update system prompts to request summaries as regular text
- Remove final_output_called field from StreamingState
- Update auto_continue tests to remove final_output_called parameter
- Remove final_output test from tool_execution_test.rs
- Update planner and flock prompts to not reference final_output
- Keep backwards-compat code in feedback_extraction.rs and task_result.rs

Scout report handback:
- Change from file-based to delimiter-based report extraction
- Scout outputs report between ---SCOUT_REPORT_START/END--- markers
- Research tool extracts content between markers, strips ANSI codes
- Add comprehensive tests for extraction and ANSI stripping

657 tests pass.

crates/g3-planner/src/llm.rs

@@ -391,7 +391,7 @@ Before making suggestions, please:
 After understanding the context, update the `{codepath}/g3-plan/new_requirements.md` file by prepending
 your refined requirements under the heading `{{{{CURRENT REQUIREMENTS}}}}`.
 
-Use final_output when you are done to indicate completion."#,
+When you are done, provide a brief summary of the refinements you made."#,
         codepath = codepath
     )
 }