feat: bell_on_complete — terminal bell when agent finishes

[teknium1]

Summary

Plays the terminal bell (\a) when the agent finishes a response. Useful for long-running tasks — switch to another window and your terminal dings when done.

Default: off. Enable with:

# ~/.hermes/config.yaml
display:
  bell_on_complete: true

Or: hermes config set display.bell_on_complete true

Why not a tool?

Replaces the approach in PR #320 (notification tool the agent calls). A config-driven bell is better because:

• Always fires — no relying on the agent to remember to call notify
• Zero overhead — no tool call, no token cost
• Works over SSH — bell propagates through the connection
• Universal — every terminal supports \a (audio bell, visual flash, taskbar highlight)

Changes

• cli.py: Read bell_on_complete from config, fire \a after response box
• hermes_cli/config.py: Added to DEFAULT_CONFIG
• cli-config.yaml.example: Documented
• website/docs/user-guide/configuration.md: Added to Display Settings

Test plan

1. hermes config set display.bell_on_complete true
2. Ask the agent something (e.g. hermes chat -q 'hello')
3. Verify terminal bell fires after the response
4. Test over SSH to confirm bell propagates

Closes #318