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-inprocess/src/structured.ts

@@ -21,6 +21,13 @@
  * returning a replacement assembly — see the waterfall composition caveat in
  * docs/architecture.md.)
  *
+ * FIXME: the whole enforcement dance above exists because the tool registry
+ * and prompt assembly are context-global. If they become per-agent or
+ * per-session scoped, a structured run just registers its own schema'd tool on
+ * the child's scope and this module reduces to the capture tool plus the
+ * turn-stop — no placeholder, no final-assembly swap, no strip-for-everyone-
+ * else, no global-registration lifetime dance.
+ *
  * A companion `agent/turn-continuation` listener stops a child's turn once its
  * output is captured — without it, the loop's default "had tool calls ⇒
  * continue" buys a wasted extra model step per structured child. It is also
@@ -65,11 +72,6 @@ export const STRUCTURED_OUTPUT_INSTRUCTION
     + `\`${STRUCTURED_OUTPUT_TOOL}\` tool with arguments matching its parameter schema exactly. `
     + 'Do not finish with a plain text answer: only the tool call counts as your result.'
 
-/** The nudge sent when a structured child finishes cleanly without calling the tool. */
-export const STRUCTURED_OUTPUT_NUDGE
-  = `You finished without calling \`${STRUCTURED_OUTPUT_TOOL}\`. `
-    + `Call \`${STRUCTURED_OUTPUT_TOOL}\` now with your final result matching its parameter schema.`
-
 /** One structured run's state: the schema to enforce and the captured value, once recorded. */
 interface RunState {
   readonly schema: StructuredOutputSchema