Read-only inventory of the add-ons compiled into the build — name, version,
id, API level, description, command prefixes — built from the extracted
`ocs_plugin_api` manifest. Opened with the `PLUGINS` / `PLUGINMANAGER` command
or the Start-page "Plugins" button.
Adds `installed_manifests()` to the plugin registry and wires the window the
same way as the existing About / Shortcuts windows.
Part of #100 (phase-1 plugin manager UI stub).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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