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-console/src/api/mod.rs

@@ -1,4 +1,4 @@
-pub mod instances;
 pub mod control;
+pub mod instances;
 pub mod logs;
 pub mod state;