feat(cli): add /usage command to display session token usage

Introduced a new command "/usage" in the CLI to show cumulative token usage for the current session. This includes details on prompt tokens, completion tokens, total tokens, API calls, and context state. Updated command documentation to reflect this addition. Enhanced the AIAgent class to track token usage throughout the session.

run_agent.py

@@ -535,6 +535,12 @@ class AIAgent:
         )
         self.compression_enabled = compression_enabled
         self._user_turn_count = 0
+
+        # Cumulative token usage for the session
+        self.session_prompt_tokens = 0
+        self.session_completion_tokens = 0
+        self.session_total_tokens = 0
+        self.session_api_calls = 0
         
         if not self.quiet_mode:
             if compression_enabled:
@@ -3105,6 +3111,11 @@ class AIAgent:
                             "total_tokens": total_tokens,
                         }
                         self.context_compressor.update_from_response(usage_dict)
+
+                        self.session_prompt_tokens += prompt_tokens
+                        self.session_completion_tokens += completion_tokens
+                        self.session_total_tokens += total_tokens
+                        self.session_api_calls += 1
                         
                         if self.verbose_logging:
                             logging.debug(f"Token usage: prompt={usage_dict['prompt_tokens']:,}, completion={usage_dict['completion_tokens']:,}, total={usage_dict['total_tokens']:,}")