review: drop the structured-output nudge; FIXME the context-global registry constraint

Two human review directives:

- No re-prompt. A structured child that finishes a turn cleanly without
  calling structured_output settles error to the parent immediately —
  readResult already carried that mapping; the nudge loop only delayed it.
  Deletes the loop, its cancellation-window guard, STRUCTURED_OUTPUT_NUDGE,
  and the structuredNudgeRetries Config on both backends.

- FIXME in the structured module doc: per-agent/per-session tool registry and
  prompt assembly would dissolve the final-assembly enforcement dance (the
  placeholder tool, the swap, the strip, the global-registration lifetime).

packages/subagent/subagent-spawn/tests/harness.ts

@@ -34,7 +34,7 @@ export async function spawnHarness(workdir: string): Promise<Context> {
   await ctx.plugin(LocalBashExecutor, { cwd: workdir, timeoutMs: 30_000 })
   await ctx.plugin(ToolBash)
   await ctx.plugin(SubagentService)
-  await ctx.plugin(Spawn, { providerName: 'spawn', structuredNudgeRetries: 1 })
+  await ctx.plugin(Spawn, { providerName: 'spawn' })
   // The model-facing subagent tool, bound to the spawn backend.
   await ctx.plugin(ToolSubagent, { provider: 'spawn' })
   return ctx

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', structuredNudgeRetries: 1 })
+  await ctx.plugin(spawn, { providerName: 'spawn' })
   ctx.llm.registerAdapter(['mock'], adapter)
   const parent = ctx.agentLoop.create(AgentId('parent'), { model: 'mock' })
   return { ctx, parent, adapter }
@@ -257,7 +257,7 @@ describe('dsh-subagent-spawn', () => {
     // 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 })
+    const fiber = await ctx.plugin(spawn, { providerName: 'spawn' })
     expect(ctx.subagents.list()).toEqual(['spawn'])
     await fiber.dispose()
     expect(ctx.subagents.list()).toEqual([])