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-core/src/streaming.rs

@@ -22,7 +22,6 @@ pub struct StreamingState {
     pub response_started: bool,
     pub any_tool_executed: bool,
     pub auto_summary_attempts: usize,
-    pub final_output_called: bool,
     pub turn_accumulated_usage: Option<g3_providers::Usage>,
 }
 
@@ -36,7 +35,6 @@ impl StreamingState {
             response_started: false,
             any_tool_executed: false,
             auto_summary_attempts: 0,
-            final_output_called: false,
             turn_accumulated_usage: None,
         }
     }