docs(plugin): rewrite architecture + template for the external-only model

OpenCADStudio ships no built-in plugins, so the spec and scaffold no
longer describe the in-tree inventory path, demo_plugin, or the fictional
Storm Sewer reference. The architecture doc now covers the external cdylib
model end to end (ocs_plugin_api contract, export_plugin!, building +
publishing per-platform releases, runtime loading, the marketplace +
curated registry, the approach-B ABI caveat). The template becomes a
standalone cdylib crate (Cargo.toml + src/lib.rs + release workflow)
instead of an in-tree module.

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

docs/plugin-template/src/lib.rs

@@ -0,0 +1,68 @@
+//! Open CAD Studio add-on template.
+//!
+//! Rename the crate (`Cargo.toml`), the ids/strings below, and `plugin.toml` to
+//! match. Build with `cargo build --release` and ship the resulting cdylib plus
+//! `plugin.toml` as GitHub Release assets (see `.github/workflows/release.yml`).
+
+use ocs_plugin_api::host::{BuiltinPlugin, HostApi};
+use ocs_plugin_api::manifest::{ApiVersion, PluginManifest};
+use ocs_plugin_api::ribbon::{CadModule, IconKind, ModuleEvent, RibbonGroup, RibbonItem, ToolDef};
+
+// Keep these fields in sync with `plugin.toml`.
+static MANIFEST: PluginManifest = PluginManifest {
+    id: "opencad.my_plugin",
+    name: "My Plugin",
+    version: "0.1.0",
+    description: "What this plugin does.",
+    api_version: ApiVersion::CURRENT,
+    ribbon_order: 50,
+    xdata_apps: &[],
+    command_prefixes: &["MP_"],
+};
+
+/// The ribbon tab.
+struct MyModule;
+
+impl CadModule for MyModule {
+    fn id(&self) -> &'static str {
+        "my_plugin"
+    }
+    fn title(&self) -> &'static str {
+        "My Plugin"
+    }
+    fn ribbon_groups(&self) -> Vec<RibbonGroup> {
+        vec![RibbonGroup {
+            title: "Tools",
+            tools: vec![RibbonItem::LargeTool(ToolDef {
+                id: "MP_HELLO",
+                label: "Hello",
+                icon: IconKind::Glyph("★"),
+                event: ModuleEvent::Command("MP_HELLO".to_string()),
+            })],
+        }]
+    }
+}
+
+/// The plugin entry point.
+struct MyPlugin;
+
+impl BuiltinPlugin for MyPlugin {
+    fn manifest(&self) -> &'static PluginManifest {
+        &MANIFEST
+    }
+    fn ribbon(&self) -> Box<dyn CadModule> {
+        Box::new(MyModule)
+    }
+    fn dispatch(&self, host: &mut dyn HostApi, cmd: &str) -> bool {
+        match cmd {
+            "MP_HELLO" => {
+                host.push_info("Hello from My Plugin");
+                true
+            }
+            _ => false,
+        }
+    }
+}
+
+// Emit the C-ABI symbols the host loader looks for.
+ocs_plugin_api::export_plugin!(MyPlugin);