feat(plugins): add community plugin discovery

[wandaocheng1997]

What changed

• add a persistent community-plugin discovery action to the Plugins workspace
• link discovery to the GitHub instatic-plugin topic
• improve the fresh-install empty state with a safe download-and-review workflow
• cover the external directory action

Why

The plugin system supported local package upload but offered no product path for discovering packages.

User impact

Operators can find community release ZIPs from the Plugins page, then install them through the existing package inspection and permission-review flow. No remote package is installed automatically.

Verification

• bun test src/__tests__/plugins/pluginsAdmin.test.tsx
• bun run lint
• bun run build
• relevant architecture tests