add context window monitor

Writes the current context window to logs/current_context_window (uses a symlink to a session ID).

This PR was unfortunately generated by a different LLM and did a ton of superficial reformating, it's actually a fairly small and benign change, but I don't want to roll back everything. Hope that's ok.

crates/g3-cli/src/machine_ui_writer.rs

@@ -87,9 +87,9 @@ impl UiWriter for MachineUiWriter {
     fn flush(&self) {
         let _ = io::stdout().flush();
     }
-    
+
     fn wants_full_output(&self) -> bool {
-        true  // Machine mode wants complete, untruncated output
+        true // Machine mode wants complete, untruncated output
     }
 
     fn prompt_user_yes_no(&self, message: &str) -> bool {