feat(subagent): add explicit child reports

docs/core-data-structures/subagent.md

@@ -4,7 +4,7 @@ English | [中文](subagent.zh.md)
 
 The subagent seam — an agent delegating work to a child agent. Like [bash](bash.md) it is **one optional capability**, not part of the agent-loop spine, so its vocabulary lives here rather than in [core.md](core.md). But it differs from every other seam on one axis: **multiple provider implementations coexist** in one context, registered by name (`ctx.subagents`), where bash allows only one executor. The registry shape mirrors the [LLM adapter registry](llm-streaming.md), not the single-service bash executor.
 
-Interface: [dsh-subagent](../../packages/subagent/subagent) (`ctx.subagents` + the vocabulary below). Implementations are sibling packages (`dsh-subagent-spawn`, `-fork`, `-acp`); the model-facing consumers are [dsh-tool-subagent](../../packages/subagent/tool-subagent) (per-provider delegation) and [dsh-tool-subagent-control](../../packages/subagent/tool-subagent-control) (the optional global `send_message`). The same `ctx.subagents` service owns continuable-child orchestration through an internal activation manager. The rationale lives in [the subagent Agent Note](../../.agents/notes/implemented/feature/2026-06-21-subagent-capability-seam.md), [the continuable subagents Agent Note](../../.agents/notes/implemented/feature/2026-07-28-continuable-subagent-conversations.md), and [the merged-service Agent Note](../../.agents/notes/implemented/simplification/2026-07-26-merge-subagent-control-service.md).
+Interface: [dsh-subagent](../../packages/subagent/subagent) (`ctx.subagents` + the vocabulary below). Implementations are sibling packages (`dsh-subagent-spawn`, `-fork`, `-acp`); the model-facing consumers are [dsh-tool-subagent](../../packages/subagent/tool-subagent) (per-provider delegation), [dsh-tool-subagent-control](../../packages/subagent/tool-subagent-control) (the optional global `send_message` and `list_agents` controls), and [dsh-tool-subagent-report](../../packages/subagent/tool-subagent-report) (the optional child-scoped `report` return channel). The same `ctx.subagents` service owns continuable-child orchestration through an internal activation manager and read-only direct-child discovery through optional session query. The rationale lives in [the subagent Agent Note](../../.agents/notes/implemented/feature/2026-06-21-subagent-capability-seam.md), [the continuable subagents Agent Note](../../.agents/notes/implemented/feature/2026-07-28-continuable-subagent-conversations.md), [the report-tool Agent Note](../../.agents/notes/implemented/feature/2026-07-30-continuable-subagent-report-tool.md), [the durable catalog Agent Note](../../.agents/notes/implemented/feature/2026-07-22-durable-subagent-catalog-and-list-agents.md), and [the merged-service Agent Note](../../.agents/notes/implemented/simplification/2026-07-26-merge-subagent-control-service.md).
 
 Sources: [`packages/subagent/subagent/src/types.ts`](../../packages/subagent/subagent/src/types.ts), [`packages/subagent/subagent/src/index.ts`](../../packages/subagent/subagent/src/index.ts), and [`packages/subagent/subagent/src/continuation.ts`](../../packages/subagent/subagent/src/continuation.ts)
 
@@ -174,6 +174,34 @@ interface ContinuableStart {
 }
 ```
 
+An optional continuable-child setup contribution can install scope-local capabilities after base child composition and before Activation publication. The registry is ordered and transactional: a failed or revoked setup rolls back the unpublished Activation, child-scope disposal releases every installation, new registrations affect the next Activation, and registration removal revokes every resident installation immediately.
+
+`SubagentService.reportFrom()` uses that extension seam without adding a second queue or a result-bearing child wrapper. The exact live child Agent authorizes the call; callers cannot name a recipient. The manager derives the only recipient from the child's durable `parentSession`, requires that parent Agent to be live, frames the selected content as one `subagent-report` user message, and returns the message's stable `MessageId`. Quiet delivery uses `Agent.inject()` and creates no inbox occurrence or parent turn; waking delivery uses `Agent.followup()` and creates one ordinary later parent turn. Neither mode concludes the child's turn, and no final answer reports implicitly.
+
+```ts type-equiv
+/** Durable attribution for a continuable child's explicit parent report. */
+interface SubagentReportMessageSource {
+  readonly kind: 'subagent-report'
+  /** Session id of the reporting child. */
+  readonly senderSessionId: SessionId
+}
+```
+
+```ts type-equiv
+/** Deployment scheduling policy for accepted child reports. */
+type SubagentReportDelivery = 'quiet' | 'wakeup'
+```
+
+```ts type-equiv
+/** Options for one continuable child's report to its direct parent. */
+interface SubagentReportOptions {
+  /** Already-resolved parent scheduling policy. */
+  readonly delivery: SubagentReportDelivery
+  /** Caller cancellation, owning authorization and admission until acceptance. */
+  readonly signal: AbortSignal
+}
+```
+
 The provider participates only in preparing the initial creation spec, where `spawn` and `fork` differ. Its returned spec carries only detached provider-specific creation inputs — today the optional parent-history seed — and no Agent, `AgentHandle`, prompt delivery, result, disposal, or resume operation. Cold resume does not dispatch through a provider at all: the manager folds the generic descriptor, calls `ctx.agents.resume()` through the same activation-owner scope, and submits the waiting turn.
 
 ```ts type-equiv