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-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', structuredNudgeRetries: 1 })
-  await ctx.plugin(fork, { providerName: 'fork', structuredNudgeRetries: 1 })
+  await ctx.plugin(Spawn, { providerName: 'spawn' })
+  await ctx.plugin(fork, { providerName: 'fork' })
   ctx.llm.registerAdapter(['mock'], new MockAdapter(script))
   const parent = ctx.agentLoop.create(AgentId('parent'), { model: 'mock' })
   return { ctx, parent }