Structured output on the subagent seam: schema subset, capture runtime, spawn/fork support

Carved out of #170 per review feedback — the foundation the workflow tool
builds on, now standing alone on master:

- dsh-tools: the structured-output JSON Schema subset (StructuredOutputSchema,
  assertSupportedOutputSchema, validateStructuredValue) — rejects loud outside
  the enforced subset, listing every violation
- dsh-subagent: SubagentStartRequest.outputSchema / SubagentResult.structured
  become a real capability; the service rejects a schema'd request whose
  provider lacks it
- dsh-subagent-inprocess: the shared structured runtime — one global
  structured_output capture tool, a prepend final-assembly listener that
  strips the placeholder for plain agents and swaps in the run's own schema
  (plus the calling instruction as a trailing section) for structured
  children, an agent/turn-continuation veto once captured, and the
  capture/nudge loop in the run driver (structuredNudgeRetries, cancellation
  honored mid-nudge); lifetime refcounted by backends and live runs
- subagent-spawn / subagent-fork flip outputSchema: true

One deliberate divergence from the #170 revision: the backends do NOT add
'tools' to their plugin inject. Doing so deferred their apply past the todo
plugin, and the delegation tool mirrors provider lifecycle — so the
model-visible tool order of every existing prompt changed, invalidating every
recorded snapshot fixture. The runtime now gates its capture-tool registration
on tools availability itself (sync when live, a scoped inject fiber when the
Loader starts the backend first), keeping this PR byte-invisible to existing
transcripts: all 35 snapshot scenarios pass against master's fixtures
unchanged.

packages/core/tools/src/index.ts

@@ -28,6 +28,16 @@ export {
   type JsonSchemaObject,
 } from './schema.ts'
 
+export {
+  assertSupportedOutputSchema,
+  validateStructuredValue,
+  OutputSchemaError,
+  type StructuredOutputSchema,
+  type StructuredSchemaNode,
+  type StructuredSchemaType,
+  type StructuredScalar,
+} from './json-schema.ts'
+
 // The render-intent vocabulary a tool declares via `presentCall`/`presentResult`
 // lives in its own UI-facing module; re-export it so `@deepseek-ai/dsh-tools`
 // stays the single public surface for consumers (producers + the ACP bridge).