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-core/examples/test_python_query.rs

@@ -1,7 +1,7 @@
 //! Test Python async query
 
-use tree_sitter::{Parser, Query, QueryCursor, Language};
 use streaming_iterator::StreamingIterator;
+use tree_sitter::{Language, Parser, Query, QueryCursor};
 
 fn main() -> anyhow::Result<()> {
     let source_code = r#"