feat: support external emulator plugins

[jsj]

Summary

Adds a small external-plugin loading path on top of the existing ServicePlugin architecture.

This keeps built-in emulators unchanged, but lets teams run emulator packages that are not ready or appropriate for core yet:

npx emulate start --plugin ./plugins/my-service.ts --service my-service
npx emulate init --plugin ./plugins/my-service.ts --service my-service
npx emulate list --plugin ./plugins/my-service.ts

Programmatic usage also works:

await createEmulator({
  service: "my-service",
  plugins: ["./plugins/my-service.ts"],
});

Why this matters

The volume of emulator PRs since this was opened is the signal: service coverage is valuable, but every new provider currently has to land as a core registry/package change. That makes the project carry every niche, experimental, or company-specific emulator forever.

This PR gives maintainers a lower-risk answer:

• widely useful emulators can still be built in;
• experimental or private emulators can live outside this repo;
• users still get the same start, init, list, seed config, auth fallback, and programmatic APIs.

Low-risk shape

• Built-in service behavior is preserved.
• Existing ServicePlugin is reused; no new runtime abstraction.
• External plugins are opt-in via --plugin / plugins.
• External plugin names cannot collide with built-ins.
• Duplicate external plugin names are rejected.
• Local paths and package names are both supported.
• init all includes external plugins only when explicitly supplied.

Plugin contract

A plugin module exports a ServicePlugin as either plugin or default export.

Optional exports:

• seedFromConfig
• label
• endpoints
• defaultFallback
• initConfig

The tests include a tiny echo-plugin.ts fixture to make the contract concrete and reviewable.

Test plan

• pnpm --filter emulate type-check
• pnpm --filter emulate test
• pnpm --filter emulate lint

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