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/external.rs

@@ -205,7 +205,7 @@ fn parse_string_array(s: &str) -> Vec<String> {
 // ── Runtime loading (desktop only) ──────────────────────────────────────────
 
 #[cfg(not(target_arch = "wasm32"))]
-pub use loader::{load, load_at_startup, loaded_ids, with_loaded, LoadedPlugin};
+pub use loader::{load_at_startup, loaded_ids, with_loaded};
 
 #[cfg(not(target_arch = "wasm32"))]
 mod loader {