feat: bell_on_complete — terminal bell when agent finishes

Adds a simple config option to play the terminal bell (\a) when the
agent finishes a response. Useful for long-running tasks — switch to
another window and your terminal will ding when done.

Works over SSH since the bell character propagates through the
connection. Most terminal emulators can be configured to flash the
taskbar, play a sound, or show a visual indicator on bell.

Config (default: off):
  display:
    bell_on_complete: true

Closes #318

hermes_cli/config.py

@@ -107,6 +107,7 @@ DEFAULT_CONFIG = {
         "compact": False,
         "personality": "kawaii",
         "resume_display": "full",  # "full" (show previous messages) | "minimal" (one-liner only)
+        "bell_on_complete": False,  # Play terminal bell (\a) when agent finishes a response
     },
     
     # Text-to-speech configuration