[FEATURE]: Add persistent ui.sidebar.enabled config to disable Context sidebar

The Context sidebar in the opencode TUI keeps reappearing after restart. There's no persistent configuration option to disable it.

Requested feature:
Add a config option in opencode.json:

{
  "ui": {
    "sidebar": {
      "enabled": false
    }
  }
}

Current behavior:

• Sidebar toggles with Ctrl+B but resets on restart
• No ui.sidebar.enabled option in config schema
• Context panel (from FlowDeck's context-steward skill) persists across sessions

Expected behavior:

• Setting ui.sidebar.enabled: false keeps sidebar hidden permanently
• Respects user preference across restarts

Environment:

• opencode version: [run opencode --version]
• OS: Linux
• Config: project-level opencode.json

• I have searched existing issues and confirmed this feature has not been suggested before