Move T3 sidebar plugin to examples

[SawyerHood]

Summary

• Move the T3 sidebar plugin from plugins/ to examples/plugins/.
• Remove T3 Sidebar from the bundled official plugin catalog.
• Add local icon, select, and style helpers so the example does not use BB's private shared UI package.
• Update the plugin guides, reference documents, catalog tests, generated templates, and lockfile.

Why

T3 Sidebar is a reference implementation for the sidebar replacement APIs. It should remain an installable example instead of a bundled product plugin.

Impact

New BB builds will not include T3 Sidebar in the official plugin catalog. Developers can install it from a checkout with:

bb plugin install ./examples/plugins/t3sidebar

Validation

• pnpm exec turbo run typecheck --filter=bb-plugin-t3sidebar --filter=@bb/server --filter=@bb/templates
• pnpm exec turbo run test --filter=bb-plugin-t3sidebar --force
• pnpm exec vitest run --config vitest.config.ts test/services/plugins/official-plugins.test.ts test/services/plugins/builtin-plugins.test.ts
• pnpm run bb -- plugin build examples/plugins/t3sidebar