docs: add opencode-notifier plugin to ecosystem

[mohak34]

Adds the opencode-notifier plugin to the ecosystem page. This plugin provides:

• Desktop notifications for permission requests, session completion, and errors
• Configurable sound alerts for each event type
• Cross-platform support (macOS, Linux, Windows)
• Per-event toggles for notifications and sounds

Note

I noticed that opencode-notificator (line 35) returns a 404 - the repository appears to be unavailable.