This switches TUI plugin mentions to use app-server `plugin/list` for
plugin inventory and metadata instead of `PluginManager`, while keeping
the same mention-eligibility filters as before.
Same filters as before:
- Only plugins in the current config / cwd scope.
- Only installed and enabled plugins.
- Only plugins that actually expose a capability, meaning at least one
skill, MCP server, or app connector.
- Uses `plugin/list` for the mention names/descriptions