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/src/ui_writer.rs

@@ -66,10 +66,6 @@ pub trait UiWriter: Send + Sync {
     /// Returns the index of the selected option
     fn prompt_user_choice(&self, message: &str, options: &[&str]) -> usize;
 
-    /// Print the final output summary with markdown formatting
-    /// Shows a spinner while formatting, then renders the markdown
-    fn print_final_output(&self, summary: &str);
-
     /// Filter JSON tool calls from streaming content for display.
     /// This is a UI concern - the raw content should be preserved for logging.
     /// Default implementation passes through unchanged.
@@ -125,7 +121,4 @@ impl UiWriter for NullUiWriter {
     fn prompt_user_choice(&self, _message: &str, _options: &[&str]) -> usize {
         0
     }
-    fn print_final_output(&self, _summary: &str) {
-        // No-op for null writer
-    }
 }