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/simple_output.rs

@@ -6,7 +6,9 @@ pub struct SimpleOutput {
 
 impl SimpleOutput {
     pub fn new() -> Self {
-        SimpleOutput { machine_mode: false }
+        SimpleOutput {
+            machine_mode: false,
+        }
     }
 
     pub fn new_with_mode(machine_mode: bool) -> Self {