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-planner/src/llm.rs

@@ -304,9 +304,6 @@ impl g3_core::ui_writer::UiWriter for PlannerUiWriter {
         0 // Default to first option
     }
     
-    fn print_final_output(&self, summary: &str) {
-        println!("\n📝 Final Output:\n{}", summary);
-    }
 }
 
 /// Call LLM to refine requirements using a full Agent with tool execution