ci: use packaged dist/index.js for host e2e instead of raw TS source

Smoke workflow `smoke-opencode-linux.yml` (just added) proved that
opencode-on-Linux is healthy in isolation: both curl and bun's fetch
reach /doc in seconds with either 127.0.0.1 or 0.0.0.0 binding. So
the host e2e hang is NOT a Linux opencode bug.

The remaining structural difference between smoke (~10s spawn-to-OK)
and host e2e (~5min timeout) is plugin loading. Host e2e was pointing
opencode at `packages/plugin/src/index.ts` — raw TypeScript source
spanning hundreds of submodule imports. Bun's runtime TS transpile +
dynamic resolution can be slow on cold Linux runners.

Production never loads from src/ — published users get a single
bundled `dist/index.js`. Testing src/ exposes us to a slowness path
real users never see. CI workflows already build dist before host
e2e, so the file is available; the harness just wasn't using it.

Resolution: prefer `packages/plugin/dist/index.js` when it exists,
fall back to `src/` only when running locally without a build (which
also avoids breaking local devs who haven't built).