feat(core): explicit plugins.register API (replaces no-op autoLoad/directory)

[lane711]

Summary

• Adds plugins.register?: Plugin[] to SonicJSConfig so user plugins are mounted by createSonicJSApp directly — no more wrapping the core app in a custom Hono() instance just to attach plugin routes.
• Marks plugins.directory / plugins.autoLoad as @deprecated no-ops. They were declared in the type but never wired up; bootstrap only iterated the hardcoded core plugin registry, so user plugins silently failed to load.
• New mountPlugin() helper (packages/core/src/plugins/mount.ts) sorts middleware by priority, then mounts routes. User plugins are registered before /admin/plugins and the /admin catch-all so plugin admin pages aren't shadowed.

Why explicit, not filesystem autoload

SonicJS runs on Cloudflare Workers — no runtime fs, so any directory autoload would have to be a build-time Vite plugin. That's a lot of machinery for two lines of saved code, and most mature plugin ecosystems (Vite, Astro, Fastify, Nuxt modules) landed on explicit register: [...] arrays for the same reasons: type-safety, tree-shaking, debuggability, and the ability to pass config (weatherPlugin({ apiKey })) inline.

Originated from a Discord report: a user couldn't get a custom plugin to load with autoLoad: true. The flag did nothing.

Before / after (starter app)

// before — manual Hono wrapper, manual route loop
const coreApp = createSonicJSApp(config)
const app = new Hono()
if (contactFormPlugin.routes) {
  for (const route of contactFormPlugin.routes) app.route(route.path, route.handler)
}
app.route('/', coreApp)
export default app

// after
export default createSonicJSApp({
  collections: { autoSync: true },
  plugins: { register: [contactFormPlugin] },
})

Test plan

• npx vitest run in packages/core — 1479 passed, 328 skipped, 0 failed
• New mount.test.ts covers route mounting, multi-route, global vs scoped middleware, priority ordering, and empty-plugin no-op (6 tests)
• npx tsc --noEmit in packages/core is clean
• packages/stats still compiles (disableAll: true path unchanged)
• Visual smoke test of starter template + a custom plugin in dev — recommended before merging

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