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/support/subagent-mock/tests/subagent-mock.spec.ts

@@ -41,13 +41,13 @@ describe('dsh-subagent-mock', () => {
 
   it('surfaces a structured result when the request carries an outputSchema', async () => {
     const ctx = await mount({ reply: 'r', structured: { answer: 42 } })
-    const run = ctx.subagents.start('mock', baseRequest({ outputSchema: { answer: { type: 'number' } } }))
+    const run = ctx.subagents.start('mock', baseRequest({ outputSchema: { type: 'object', properties: { answer: { type: 'number' } } } }))
     await expect(run.result).resolves.toMatchObject({ structured: { answer: 42 } })
   })
 
   it('defaults structured output to { reply } when outputSchema is requested but no structured value is configured', async () => {
     const ctx = await mount({ reply: 'fallback reply' })
-    const run = ctx.subagents.start('mock', baseRequest({ outputSchema: { answer: { type: 'number' } } }))
+    const run = ctx.subagents.start('mock', baseRequest({ outputSchema: { type: 'object', properties: { answer: { type: 'number' } } } }))
     await expect(run.result).resolves.toMatchObject({ structured: { reply: 'fallback reply' } })
   })