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/subagent/subagent-spawn/tests/subagent-spawn.spec.ts

@@ -34,7 +34,7 @@ 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(spawn, { providerName: 'spawn', structuredNudgeRetries: 1 })
   ctx.llm.registerAdapter(['mock'], adapter)
   const parent = ctx.agentLoop.create(AgentId('parent'), { model: 'mock' })
   return { ctx, parent, adapter }
@@ -241,17 +241,23 @@ describe('dsh-subagent-spawn', () => {
     await parentHandle.dispose()
   })
 
-  it('advertises depthLimit but not outputSchema/toolFilter', async () => {
+  it('advertises depthLimit and outputSchema but not toolFilter', async () => {
     const { ctx } = await setup([])
     const provider = ctx.subagents.getProvider('spawn')!
-    expect(provider.capabilities).toEqual({ outputSchema: false, depthLimit: true, toolFilter: false })
+    expect(provider.capabilities).toEqual({ outputSchema: true, depthLimit: true, toolFilter: false })
   })
 
   it('unregisters the provider when its fiber is disposed (HMR safety)', async () => {
     const ctx = new Context()
     await ctx.plugin(SubagentService)
     await ctx.plugin(AgentRegistry)
-    const fiber = await ctx.plugin(spawn, { providerName: 'spawn' })
+    // The backend does NOT inject 'tools' (the structured runtime gates its
+    // capture-tool registration on tools availability itself, keeping backend
+    // apply timing — and the delegation tool's prompt position — unchanged);
+    // the registries are loaded here so the runtime registers eagerly anyway.
+    await ctx.plugin(SystemPrompt)
+    await ctx.plugin(ToolRegistry)
+    const fiber = await ctx.plugin(spawn, { providerName: 'spawn', structuredNudgeRetries: 1 })
     expect(ctx.subagents.list()).toEqual(['spawn'])
     await fiber.dispose()
     expect(ctx.subagents.list()).toEqual([])