fix(cli): preserve plugin commands in split bundles

[ymichael]

What was wrong

The code-split CLI dynamically imported the CommonJS undici package and requested its named Agent export, but esbuild emitted that split chunk with only a default export. Every plugin-contributed command therefore failed while constructing its dispatcher, before the server-side handler ran; ordinary core commands were unaffected.

What changed

The plugin proxy now statically imports Agent inside its already lazy-loaded module and caches the dispatcher synchronously, preserving bundler interop without adding undici to core-command startup. The startup-graph suite now executes both a direct plugin command and bb plugin run from a real split artifact against a small HTTP fixture. No CLI contract, documentation, server/daemon wire behavior, or HOST_DAEMON_PROTOCOL_VERSION change is required.

How you verified

The new split-artifact regression test failed before the production change with Agent is not a constructor and passes after it.

• pnpm exec turbo run test --filter=@bb/cli --force — 50 files, 476 tests passed
• pnpm exec turbo run typecheck --filter=@bb/cli --force — 4 tasks passed
• pnpm exec turbo run build --filter=bb-app --force — 11 tasks passed
• Dev and packaged split artifacts both ran direct automation --help, explicit plugin run automations --help, and core status --json successfully

Fixes #2349

AGENT GENERATED