fix(scope): close final ownership races

Drain idle injection flushes before agent teardown, snapshot approval and subagent provider inputs, and gate subagent lifecycle events on real child readiness. Align the RFCs and generated contracts with the hardened behavior.

packages/support/subagent-mock/src/index.ts

@@ -66,6 +66,9 @@ class MockSubagentProvider implements SubagentProvider {
 
     return {
       id,
+      // A scripted run has no asynchronous publication phase; it is ready as
+      // soon as the provider returns the handle.
+      started: Promise.resolve(),
       result: Promise.resolve().then(resultFor),
       cancel() {
         cancelled = true