Add /notify command for granular notification controls

[dmoliveira]

What changed

• Added new scripts/notify_command.py command backend for notification control in four levels:
  • global on/off (all)
  • channel on/off (sound, visual)
  • per-event on/off (complete, error, permission, question)
  • per-event channel toggles (channel <event> <sound|visual> <on|off>)
• Added notification profiles:
  • all, quiet, focus, sound-only, visual-only
• Added OpenCode command bindings in opencode.json:
  • /notify, /notify-help, /notify-profile-all, /notify-profile-focus, /notify-sound-only, /notify-visual-only
• Updated installer to chmod and self-check notify command
• Updated README docs with usage examples and config shape
• Extended self-test to verify notify profile/toggle behavior
• Updated make install-test to include notify status smoke check

Why

This makes notifications tunable without editing JSON manually, matching your request for controls by level (all, per type, sound, visual).

Validation

• make validate
• make selftest
• make install-test