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/README.md

@@ -11,7 +11,7 @@ Packages are grouped by modular role at `packages/<group>/<pkg>/`. The group dir
 | [`core/`](core/README.md) | Product API spine: session, system-prompt, tools, agent, and the concrete loop | Product — stable surface |
 | [`llm/`](llm/README.md) | LLM capability family: the abstract service + provider adapters | Product — stable surface |
 | [`bash/`](bash/README.md) | Bash capability family: the executor seam, a local impl, and the model-facing tool | Product — stable surface |
-| [`code-runtime/`](code-runtime/README.md) | Code-execution capability family: the abstract runtime seam for model-written programs | Product — stable surface |
+| [`code-runtime/`](code-runtime/README.md) | Code-execution capability family: the abstract runtime seam for model-written programs + a worker-thread backend | Product — stable surface |
 | [`fs/`](fs/README.md) | Filesystem capability family: the abstract seam, a local impl, and the model-facing file tools | Product — stable surface |
 | [`compact/`](compact/README.md) | Compaction capability family: the abstract seam + a basic backend (tool deferred) | Product — stable surface |
 | [`subagent/`](subagent/README.md) | Subagent capability family: the provider-registry seam and the model-facing delegation tool | Product — stable surface |