feat(core): wire v2 subagent tool (#34320)

packages/core/src/session/execution.ts

@@ -15,6 +15,8 @@ export interface Interface {
   readonly wake: (sessionID: SessionSchema.ID) => Effect.Effect<void>
   /** Interrupt active work owned by this process. Idle interruption is a no-op. */
   readonly interrupt: (sessionID: SessionSchema.ID) => Effect.Effect<void>
+  /** Resolves once this process owns no active execution for the Session. Returns immediately when idle and never starts work. */
+  readonly awaitIdle: (sessionID: SessionSchema.ID) => Effect.Effect<void>
 }
 
 /** Routes execution from a Session ID to the runner owned by that Session's Location. */
@@ -30,5 +32,6 @@ export const noopLayer = Layer.succeed(
     resume: () => Effect.void,
     wake: () => Effect.void,
     interrupt: () => Effect.void,
+    awaitIdle: () => Effect.void,
   }),
 )

packages/core/src/session/execution/local.ts

@@ -33,6 +33,7 @@ export const layer = Layer.effect(
       interrupt: coordinator.interrupt,
       resume: coordinator.run,
       wake: coordinator.wake,
+      awaitIdle: coordinator.awaitIdle,
     })
   }),
 )

packages/core/src/session/run-coordinator.ts

@@ -12,6 +12,8 @@ export interface Coordinator<Key, E> {
   readonly wake: (key: Key) => Effect.Effect<void>
   /** Stops active execution and waits for its cleanup. */
   readonly interrupt: (key: Key) => Effect.Effect<void>
+  /** Resolves once no execution is active for the key. Returns immediately when already idle and never starts work. */
+  readonly awaitIdle: (key: Key) => Effect.Effect<void>
 }
 
 type Entry<E> = {
@@ -100,5 +102,15 @@ export const make = <Key, E>(options: {
         return Fiber.interrupt(entry.owner)
       })
 
-    return { active: Effect.sync(() => new Set(active.keys())), run, wake, interrupt }
+    // Each successful drain reuses its entry.done across coalesced wakes, so one await
+    // already spans steered and queued continuation. Re-check after it settles to cover a
+    // fresh wake (or a failure/stopping successor) that installs a new entry.
+    const awaitIdle = (key: Key): Effect.Effect<void> =>
+      Effect.suspend(() => {
+        const entry = active.get(key)
+        if (entry === undefined) return Effect.void
+        return Deferred.await(entry.done).pipe(Effect.exit, Effect.andThen(awaitIdle(key)))
+      })
+
+    return { active: Effect.sync(() => new Set(active.keys())), run, wake, interrupt, awaitIdle }
   })