Add Bookmarks example plugin

Adds packages/plugins/examples/plugin-bookmarks-example, a new first-party
"core surface" example that exercises plugin database namespaces, local
folders, scoped API routes, and the page/sidebar/dashboard/settings UI
slots end-to-end. Registers the example as a bundled plugin in
server/src/routes/plugins.ts and excludes its workspace importer in
pnpm-workspace.yaml so PRs don't have to commit lockfile entries for it.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

packages/plugins/examples/plugin-bookmarks-example/migrations/001_bookmarks.sql

@@ -0,0 +1,16 @@
+CREATE TABLE plugin_bookmarks_b34a9f8617.bookmarks (
+  id uuid PRIMARY KEY,
+  company_id uuid NOT NULL,
+  slug text NOT NULL,
+  url text NOT NULL,
+  title text NOT NULL,
+  notes text NOT NULL DEFAULT '',
+  tags text[] NOT NULL DEFAULT '{}',
+  file_path text NOT NULL,
+  created_at timestamptz NOT NULL DEFAULT now(),
+  updated_at timestamptz NOT NULL DEFAULT now(),
+  CONSTRAINT bookmarks_company_slug_uq UNIQUE (company_id, slug)
+);
+
+CREATE INDEX bookmarks_company_idx
+  ON plugin_bookmarks_b34a9f8617.bookmarks (company_id, created_at DESC);