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-execution/examples/setup_coverage_tools.rs

@@ -3,7 +3,7 @@ use g3_execution::ensure_coverage_tools_installed;
 fn main() -> anyhow::Result<()> {
     // Ensure coverage tools are installed
     let already_installed = ensure_coverage_tools_installed()?;
-    
+
     if already_installed {
         println!("All coverage tools are already installed!");
     } else {