feat(agent): dedupe agent.spawn by normalized goal within a parent session (AG-4)

A spawn whose goal (NFKC, case, whitespace and punctuation folded) matches a
child of the same parent that is still queued/running, or succeeded/waiting
within the last 10 minutes, returns that child instead of creating a new one:
output gets reused=true, a message ("已有同目标子会话…") and the child's
current result summary. Failed/cancelled/over-budget children are not reused.
Spawn decisions are serialized per parent in-process so parallel duplicate
calls in one batch create a single child. Replays keyed by idempotency still
hit the original child as before. The subagent constraint in the system prompt
asks for Chinese sub-goals and no duplicate spawns.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

packages/contracts/src/capabilities-io.ts

@@ -390,6 +390,13 @@ export const SubagentResult = z.object({
   pending: z.boolean(),
 });
 export const AgentSpawnOutput = z.object({
+  /**
+   * AG-4 去重:同一父会话里同一目标(归一化后)的子会话还在运行或 10 分钟内已完成时不新建,
+   * 返回已有子会话(reused = true,result 带它的当前结果摘要);界面的"派出 N 个子会话"不计这次调用
+   */
+  reused: z.boolean().optional(),
+  /** 给模型的一句说明(reused 时:"已有同目标子会话…") */
+  message: z.string().optional(),
   runId: Id,
   depth: NonNegInt,
   mode: z.enum(["execute", "plan_only"]),

packages/contracts/src/capability.ts

@@ -347,7 +347,7 @@ export const CAPABILITY_CATALOG: Record<KnownCapabilityId, CapabilityDescriptor>
   "agent.spawn": d(
     "agent.spawn",
     "派生子会话",
-    "把一个有界子目标交给子会话处理：能力白名单须是本会话白名单的子集，预算从本会话剩余预算里划拨，模式不高于本会话；返回子会话 runId。await=true 时等它结束并返回汇报",
+    "把一个有界子目标交给子会话处理：能力白名单须是本会话白名单的子集，预算从本会话剩余预算里划拨，模式不高于本会话；返回子会话 runId。await=true 时等它结束并返回汇报。同一目标的子会话还在运行或 10 分钟内已完成时不新建，直接返回已有子会话（reused=true）",
     "propose",
     "AgentSpawn",
     "organize.execute",