worker-shell: sqlite3 is unusable in the published package — sqlite3-worker.js is referenced but its module content is not in the tarball

## Summary

The `sqlite3` shell feature group can be imported and wired up per the README, but every invocation fails at runtime:

```
sqlite3: worker error: sqlite3 worker failed to load: sqlite3 worker not found. Run 'pnpm build' to compile the worker.
```

The error tells the user to run `pnpm build`, which isn't actionable from an installed npm package — it reads like a build step that didn't run before publish.

## Reproduce

Clean install, nothing else in the picture:

```bash
npm init -y
npm install @cloudflare/computer@0.2.0
```

Wire the group into `WorkerShellBackend` as documented:

```ts
import sqliteModules from "@cloudflare/computer/shell/sqlite";

new WorkerShellBackend({ /* … */ commands: [sqliteModules] })
```

Then:

```bash
sqlite3 /demo/app.db "CREATE TABLE t(k TEXT); SELECT 1;"
# exitCode 1
# stderr: sqlite3 worker failed to load: sqlite3 worker not found. Run 'pnpm build' to compile the worker.
```

Other feature groups from the same mechanism work fine — `curl` and `jq` both behave as documented (verified against a live API), so this looks specific to the sqlite group rather than to how the groups are wired.

## Diagnosis

In the published tarball, `dist/backends/worker-shell/shell/sqlite.js` **references** `sqlite3-worker.js` but never **defines** it. Against a clean `npm install` of `0.2.0`:

```
references to sqlite3-worker.js: 1
has module content key ("sqlite3-worker.js": { "js": …): false
error string present: true
```

Every other module in that file ships as a `".js": { "js": "…" }` entry in the inlined module map — including the sql.js WASM payload, which is present. The `sqlite3-worker.js` entry point is the one piece missing, so the loader looks it up, misses, and hits the `pnpm build` branch.

So the WASM made it into the tarball but its worker entry point didn't — consistent with a build/publish step that doesn't emit (or doesn't include) that artifact.

## Environment

- `@cloudflare/computer@0.2.0`
- `wrangler@4.123.0`, `workerd@1.20260811.1`
- macOS 25.2.0, local `wrangler dev`
- Backend: worker-shell (isolate), `nodejs_compat` + `experimental` + `enable_ctx_exports`

## Note

If `sqlite3` is known-incomplete in this preview, dropping a line in the README's feature-group list would be enough — the current failure mode points at a `pnpm build` that an npm consumer can't run.