feat: royal blue tool names in agent mode + fix README heading display

- Add set_agent_mode() to UiWriter trait for visual mode differentiation
- ConsoleUiWriter uses royal blue (ANSI 256 color 69) for tool names in agent mode
- Fix extract_readme_heading() to search only README section of combined content
  (was incorrectly showing AGENTS.md heading instead of README heading)

crates/g3-core/src/ui_writer.rs

@@ -81,6 +81,11 @@ pub trait UiWriter: Send + Sync {
     /// Called at the start of a new response to clear any partial state.
     /// Default implementation does nothing.
     fn reset_json_filter(&self) {}
+
+    /// Set whether the UI is in agent mode.
+    /// When in agent mode, tool names may be displayed differently (e.g., different color).
+    /// Default implementation does nothing.
+    fn set_agent_mode(&self, _is_agent_mode: bool) {}
 }
 
 /// A no-op implementation for when UI output is not needed