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.

hermes_cli/commands.py

@@ -27,6 +27,7 @@ COMMANDS = {
     "/platforms": "Show gateway/messaging platform status",
     "/verbose": "Cycle tool progress display: off → new → all → verbose",
     "/compress": "Manually compress conversation context (flush memories + summarize)",
+    "/usage": "Show token usage for the current session",
     "/quit": "Exit the CLI (also: /exit, /q)",
 }