feat: add the worker-thread code runtime (dsh-code-runtime-worker)

The shipped backend of the code-execution seam, per the Code Mode RFC's
worker-thread section: one fresh Node worker per run, executing the
model's TypeScript after a host-side type-strip (wrapped in an
async-function shell so top-level return/await parse, sliced back out
position-preserved), bindings bridged over the message port under
hostile-peer rules (own-property name lookup, at-most-once replies,
post-settlement drops, null-prototype namespaces), logs streamed eagerly
with an in-band truncation marker, and two independent budgets — measured
event-loop busy time (computeMs) plus a never-pausing wall ceiling
(maxWallMs) — funneling into worker.terminate(). env: {} and execArgv: []
keep the isolate hermetic; disposal aborts in-flight runs and awaits
worker exits.

The worker entry loads unbuilt via Node's native type stripping
(src/worker.ts, erasable-only) and ships built as a sibling tsdown bundle
(lib/worker.js); tests/built-lib.e2e.ts pins the built load path under
plain node and joins the built-artifact smoke gate. Unit suites cover the
bootstrap in-process (fake port) and the runtime over real workers,
per-file 100%.

packages/code-runtime/code-runtime-worker/src/worker.ts

@@ -0,0 +1,20 @@
+/**
+ * The worker-thread entrypoint: self-executing glue over
+ * `bootstrap.ts`'s {@link runWorkerMain}, kept to the spawn wiring alone.
+ * Like `bin.ts` CLI entrypoints, this file executes only inside a spawned
+ * worker isolate — a place the coverage provider cannot observe — so it is
+ * excluded from the coverage gate while every line of actual logic lives in
+ * `bootstrap.ts`, unit-tested in-process; the real spawn path is pinned by
+ * the integration tests that run genuine workers.
+ *
+ * @module @deepseek-ai/dsh-code-runtime-worker/src/worker
+ */
+
+import { parentPort, workerData } from 'node:worker_threads'
+import { runWorkerMain } from './bootstrap.ts'
+import type { WorkerBootData } from './protocol.ts'
+
+// A worker always has a parent port; guard loudly rather than run detached.
+if (!parentPort) throw new Error('dsh-code-runtime-worker: worker entry loaded outside a worker thread')
+
+await runWorkerMain(parentPort, workerData as WorkerBootData, { stdout: process.stdout, stderr: process.stderr })