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/.github/workflows/release.yml

@@ -0,0 +1,44 @@
+name: release
+
+# Build the plugin cdylib for each desktop platform and attach the binaries
+# (plus plugin.toml) to a GitHub Release on a v* tag. Open CAD Studio's Plugin
+# Manager reads the assets, picks the one matching the user's OS/arch, and
+# installs it. Rename the `asset:` files to your plugin id.
+on:
+  push:
+    tags: ["v*"]
+
+permissions:
+  contents: write
+
+jobs:
+  build:
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - os: ubuntu-latest
+            ext: so
+            asset: opencad.my_plugin-linux-x86_64.so
+          - os: windows-latest
+            ext: dll
+            asset: opencad.my_plugin-windows-x86_64.dll
+          - os: macos-latest
+            ext: dylib
+            asset: opencad.my_plugin-macos-aarch64.dylib
+    runs-on: ${{ matrix.os }}
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
+      - run: cargo build --release
+      - name: Stage release asset
+        shell: bash
+        run: |
+          mkdir -p dist
+          cp target/release/*.${{ matrix.ext }} "dist/${{ matrix.asset }}"
+          cp plugin.toml dist/plugin.toml
+      - uses: softprops/action-gh-release@v2
+        with:
+          files: |
+            dist/${{ matrix.asset }}
+            dist/plugin.toml