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

cli-config.yaml.example

@@ -635,3 +635,8 @@ display:
   #   verbose: Full args, results, and debug logs (same as /verbose)
   # Toggle at runtime with /verbose in the CLI
   tool_progress: all
+
+  # Play terminal bell when agent finishes a response.
+  # Useful for long-running tasks — your terminal will ding when the agent is done.
+  # Works over SSH. Most terminals can be configured to flash the taskbar or play a sound.
+  bell_on_complete: false