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/ui_writer.rs

@@ -4,58 +4,60 @@
 pub trait UiWriter: Send + Sync {
     /// Print a simple message
     fn print(&self, message: &str);
-    
+
     /// Print a message with a newline
     fn println(&self, message: &str);
-    
+
     /// Print without newline (for progress indicators)
     fn print_inline(&self, message: &str);
-    
+
     /// Print a system prompt section
     fn print_system_prompt(&self, prompt: &str);
-    
+
     /// Print a context window status message
     fn print_context_status(&self, message: &str);
-    
+
     /// Print a context thinning success message with highlight and animation
     fn print_context_thinning(&self, message: &str);
-    
+
     /// Print a tool execution header
     fn print_tool_header(&self, tool_name: &str);
-    
+
     /// Print a tool argument
     fn print_tool_arg(&self, key: &str, value: &str);
-    
+
     /// Print tool output header
     fn print_tool_output_header(&self);
-    
+
     /// Update the current tool output line (replaces previous line)
     fn update_tool_output_line(&self, line: &str);
-    
+
     /// Print a tool output line
     fn print_tool_output_line(&self, line: &str);
-    
+
     /// Print tool output summary (when output is truncated)
     fn print_tool_output_summary(&self, hidden_count: usize);
-    
+
     /// Print tool execution timing
     fn print_tool_timing(&self, duration_str: &str);
-    
+
     /// Print the agent prompt indicator
     fn print_agent_prompt(&self);
-    
+
     /// Print agent response inline (for streaming)
     fn print_agent_response(&self, content: &str);
-    
+
     /// Notify that an SSE event was received (including pings)
     fn notify_sse_received(&self);
-    
+
     /// Flush any buffered output
     fn flush(&self);
-    
+
     /// Returns true if this UI writer wants full, untruncated output
     /// Default is false (truncate for human readability)
-    fn wants_full_output(&self) -> bool { false }
+    fn wants_full_output(&self) -> bool {
+        false
+    }
 
     /// Prompt the user for a yes/no confirmation
     fn prompt_user_yes_no(&self, message: &str) -> bool;
@@ -86,7 +88,13 @@ impl UiWriter for NullUiWriter {
     fn print_agent_response(&self, _content: &str) {}
     fn notify_sse_received(&self) {}
     fn flush(&self) {}
-    fn wants_full_output(&self) -> bool { false }
-    fn prompt_user_yes_no(&self, _message: &str) -> bool { true }
-    fn prompt_user_choice(&self, _message: &str, _options: &[&str]) -> usize { 0 }
-}
+    fn wants_full_output(&self) -> bool {
+        false
+    }
+    fn prompt_user_yes_no(&self, _message: &str) -> bool {
+        true
+    }
+    fn prompt_user_choice(&self, _message: &str, _options: &[&str]) -> usize {
+        0
+    }
+}