refactor(plugin): drop built-in plugins, external-only

OpenCADStudio no longer bundles plugins — every add-on is an external
cdylib loaded from the plugins folder or installed via the marketplace.

- Remove the demo plugin and the in-tree ocs_example_plugin crate.
- Remove the inventory-based built-in registration path (PluginRegistration,
  all_plugins, installed_manifests); registry now serves core ribbon tabs
  plus loaded external plugins, and try_dispatch routes only to externals.
- Plugin Manager drops the 'compiled into this build' section; the
  enable/disable toggle moves onto loaded external cards.
- Prune the now-dead HostSession/DocumentTab helpers and re-exports.

Part of the #100 extensibility epic (phase 2).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/plugin/host.rs

@@ -1,9 +1,7 @@
 // Plugin traits — HostSession lives in `app::plugin_host` (same-crate field
 // access) and implements the stable `HostApi` contract plugins target.
 
-pub(crate) use crate::app::plugin_host::HostSession;
-/// The stable contract types a plugin targets. `BuiltinPlugin` (the package
-/// entry point) and `HostApi` (the runtime surface its `dispatch` receives)
-/// both live in `ocs_plugin_api` so in-tree and out-of-tree add-ons implement
-/// the same trait. See `docs/plugin-architecture.md`.
-pub use ocs_plugin_api::host::{BuiltinPlugin, HostApi};
+// The session adapter the registry wraps `app` in to dispatch to plugins. The
+// plugin-facing contract — `BuiltinPlugin` + `HostApi` — lives in
+// `ocs_plugin_api`; external cdylibs target it directly.
+pub(crate) use crate::app::plugin_host::HostSession;