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/README.md

@@ -0,0 +1,53 @@
+# @paperclipai/plugin-bookmarks-example
+
+A first-party example plugin that adds a small but real product surface to Paperclip: a
+company-scoped bookmark library backed by a plugin database namespace and a local markdown
+folder.
+
+The package is a follow-up to the core plugin host surface work and is intended as a
+focused reference for plugin authors. The orchestration smoke example remains the
+acceptance fixture for the orchestration APIs; this plugin is the user-facing
+counterpart that exercises the broader UI + storage surface.
+
+## Surfaces exercised
+
+- **Plugin database namespace.** A migration creates a `bookmarks` table inside the
+  plugin's restricted Postgres schema. The worker reads and writes only that
+  namespace; the restricted SQL helpers prevent cross-namespace access.
+- **Local folders.** A `bookmarks-root` folder declaration is configured by the
+  operator and atomically receives one markdown file per bookmark via
+  `ctx.localFolders.writeTextAtomic`. The DB row remains the source of truth so
+  list and delete operations stay correct even when the folder is unconfigured
+  or the disk write fails.
+- **Scoped API routes.** Three plugin-owned routes (`list`, `create`, `delete`)
+  declare board-or-agent auth and resolve the company from query/body
+  parameters before the worker sees them.
+- **UI extension slots.** A full plugin page, a sidebar entry, a dashboard
+  widget, and a settings page are all wired up to the same worker handlers via
+  `usePluginData` / `usePluginAction`.
+
+## Install
+
+```sh
+pnpm --filter @paperclipai/plugin-bookmarks-example build
+pnpm paperclipai plugin install ./packages/plugins/examples/plugin-bookmarks-example
+```
+
+After install, configure the plugin's `bookmarks-root` local folder under the
+plugin's Folders settings to enable the markdown sidecar files. The plugin works
+without it (rows live in the database namespace), but operators that want
+shareable, file-backed snapshots should point the folder at a checked-in
+directory.
+
+## Development
+
+```sh
+pnpm --filter @paperclipai/plugin-bookmarks-example test
+pnpm --filter @paperclipai/plugin-bookmarks-example typecheck
+pnpm --filter @paperclipai/plugin-bookmarks-example build
+```
+
+The worker code uses module-scoped handler closures populated in `setup()`,
+mirroring the orchestration smoke example, so the handlers can be invoked from
+both `ctx.data` / `ctx.actions` (UI) and `onApiRequest` (HTTP) without
+duplicating logic.