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/src/tilde_expansion_tests.rs

@@ -7,10 +7,10 @@ mod tilde_expansion_tests {
         // Test that shellexpand works
         let path_with_tilde = "~/test.txt";
         let expanded = shellexpand::tilde(path_with_tilde);
-        
+
         // Get the actual home directory
         let home = env::var("HOME").expect("HOME environment variable not set");
-        
+
         // Verify expansion happened
         assert_eq!(expanded.as_ref(), format!("{}/test.txt", home));
         assert!(!expanded.contains("~"));
@@ -20,9 +20,9 @@ mod tilde_expansion_tests {
     fn test_tilde_expansion_with_subdirs() {
         let path_with_tilde = "~/Documents/test.txt";
         let expanded = shellexpand::tilde(path_with_tilde);
-        
+
         let home = env::var("HOME").expect("HOME environment variable not set");
-        
+
         assert_eq!(expanded.as_ref(), format!("{}/Documents/test.txt", home));
     }
 
@@ -30,7 +30,7 @@ mod tilde_expansion_tests {
     fn test_no_tilde_unchanged() {
         let path_without_tilde = "/absolute/path/test.txt";
         let expanded = shellexpand::tilde(path_without_tilde);
-        
+
         assert_eq!(expanded.as_ref(), path_without_tilde);
     }
 }