subagent: implement structured output for in-process backends

The seam vocabulary (SubagentStartRequest.outputSchema, SubagentResult
.structured) existed but no in-process backend honored it — spawn/fork
advertised outputSchema: false. This lands the missing half:

- dsh-tools gains a structured-output JSON Schema subset (json-schema.ts):
  StructuredOutputSchema, assertSupportedOutputSchema (rejects loud outside
  the enforced subset, every violation listed), validateStructuredValue
  (path-qualified issues, total). outputSchema's seam type becomes this raw
  JSON-Schema subset instead of the author-facing SchemaSpec DSL — the schema
  travels verbatim to the model as a forced tool's parameters.
- dsh-subagent-inprocess gains the shared structured runtime: one global
  structured_output capture tool (placeholder parameters) + a prepend:true
  agent/request listener doing FINAL-REQUEST enforcement (strip for plain
  agents, per-run schema for structured children — survives downstream
  request-replacing listeners) + an agent/turn-continuation veto that stops
  a child's turn once captured (no wasted extra model step). Lifetime is
  refcounted by backends (plugin lifetime) AND live runs (start→settle).
- startInProcessRun drives the capture: subset asserted before the child
  exists, instruction appended to the child's system prompt, clean-finish
  nudge loop (structuredNudgeRetries, backend Config, default 1), captured
  value on result.structured; a clean finish without a capture settles
  'error' (never a silent success with a missing field).
- spawn + fork flip outputSchema: true and inject 'tools'.

packages/subagent/subagent-fork/tests/multi-subagent.spec.ts

@@ -30,8 +30,8 @@ async function setup(script: Script) {
   await ctx.plugin(Invariants)
   await ctx.plugin(AgentLoop, { agents: [] })
   await ctx.plugin(SubagentService)
-  await ctx.plugin(Spawn, { providerName: 'spawn' })
-  await ctx.plugin(fork, { providerName: 'fork' })
+  await ctx.plugin(Spawn, { providerName: 'spawn', structuredNudgeRetries: 1 })
+  await ctx.plugin(fork, { providerName: 'fork', structuredNudgeRetries: 1 })
   ctx.llm.registerAdapter(['mock'], new MockAdapter(script))
   const parent = ctx.agentLoop.create(AgentId('parent'), { model: 'mock' })
   return { ctx, parent }