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 <[email protected]>

Cargo.toml

@@ -5,7 +5,7 @@ edition = "2021"
 build = "build.rs"
 
 [workspace]
-members = ["crates/ocs_plugin_api", "crates/ocs_example_plugin"]
+members = ["crates/ocs_plugin_api"]
 
 # Embed the application icon into the Windows .exe so Explorer, the taskbar,
 # the Start-menu tile and file-association entries show it (issue #107).