fix(hub): externalize remote-docker + digitalocean sandbox backends - #299

Problem

On a published (npm i -g @madarco/agentbox) install, any host-relay git op from a remote-docker box fails:

host executor failed: Cannot find package '@agentbox/sandbox-remote-docker'
imported from .../@madarco/agentbox/runtime/hub/apps/hub/chunk-*.js

This blocks git push / gh pr (and even read-only pr list) for remote-docker boxes, so a finished branch can't leave the box through the normal path. Fixes #298.

Root cause

The hub's provider map dynamic-imports every backend (apps/hub/lib/hub-backend.ts):

const IMPORTERS: Record<ProviderKind, ...> = {
  docker:          () => import('@agentbox/sandbox-docker'),
  daytona:         () => import('@agentbox/sandbox-daytona'),
  hetzner:         () => import('@agentbox/sandbox-hetzner'),
  vercel:          () => import('@agentbox/sandbox-vercel'),
  e2b:             () => import('@agentbox/sandbox-e2b'),
  digitalocean:    () => import('@agentbox/sandbox-digitalocean'),
  'remote-docker': () => import('@agentbox/sandbox-remote-docker'),
};

But serverExternalPackages in apps/hub/next.config.mjs listed only docker/daytona/hetzner/vercel/e2b — remote-docker and digitalocean were omitted. Those two are the only providers not externalized, so on a published hub their dynamic import resolves to nothing at runtime, while the other five resolve fine.

This is the same failure class as #270 (fix(relay): inject cloud backends instead of resolving them from node_modules), which covered the CLI/relay path but not the hub's externalization list.

Fix

Add the two missing backends to serverExternalPackages so they're externalized and traced into the standalone build exactly like the other five:

     '@agentbox/sandbox-e2b',
+    '@agentbox/sandbox-digitalocean',
+    '@agentbox/sandbox-remote-docker',
   ],

Both are already declared deps of apps/hub and present in the IMPORTERS map — only the externalization entry was missing.

Validation

• node --check apps/hub/next.config.mjs passes.
• The change matches the exact, established pattern for the five working providers; the two broken ones were simply absent from the list.
• I wasn't able to run a full standalone hub build locally to assert the traced output end-to-end — CI's build should confirm. Reproduced the original failure against @madarco/agentbox 0.27.1 and 0.28.0-nightly (both ship no sandbox-remote-docker).

Suggested follow-up (not in this PR)

A small guard asserting keys(IMPORTERS) ⊆ serverExternalPackages would prevent this exact omission from recurring when a new provider is added. Happy to add it if you'd like, but apps/hub has no test harness today so I kept this PR to the one-line fix.