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/mod.rs

@@ -1,15 +1,11 @@
-// Open CAD Studio plugin runtime (phase 1: built-in, in-process).
-//
-// Generic host only — no domain logic. See `docs/plugin-architecture.md`.
-// Domain plugins (e.g. storm_sewer) live under `src/modules/<name>/` and
-// register here via `inventory::submit!(PluginRegistration { … })`.
+// Open CAD Studio plugin runtime. Plugins are external cdylibs loaded from the
+// user plugins folder (see `external`) and installed via the marketplace; the
+// host ships no built-in add-ons. See `docs/plugin-architecture.md`.
 
 pub mod external;
 pub mod host;
-pub mod manifest;
 pub mod marketplace;
 pub mod registry;
 
 pub use registry::{all_ribbon_modules, ribbon_modules_enabled};
-pub(crate) use host::BuiltinPlugin;
-pub(crate) use registry::{installed_manifests, try_dispatch};
+pub(crate) use registry::try_dispatch;