fix: verify JSON-RPC child ownership

packages/core/agent/README.md

@@ -13,6 +13,7 @@ The scoped-registration surface: `Agent.ctx` is the agent's scope context (`dsh-
 - `ctx.agents.register(agent: Agent): () => void` — record an **already-constructed** agent. Disposed with the calling fiber.
 - Advanced ordered lifecycle: `enter(agent, owner): () => void` enforces `agent.id === agent.session.id`, performs the authoritative ID collision check, and inserts without announcing; `owner` explicitly records the live creator-agent relation (or `undefined` for a root), independently of durable session lineage. `announce(agent)` emits `agent/created` exactly once. A detach requested synchronously by a creation listener is deferred until that dispatch unwinds, and every detach checks the captured entry object, so a stale capability cannot delete a later same-ID replacement. The async factory uses this split; ordinary plugins use `register()`.
 - `ctx.agents.get(id: SessionId): Agent | undefined`
+- `ctx.agents.isOwnedBy(id: SessionId, owner: Agent): boolean` — whether the exact live entry was created through that parent agent's scoped context; runtime ownership is independent of durable session lineage.
 - `ctx.agents.list(): Agent[]`
 - `ctx.agents.roots(): Agent[]` — live agents created without an owning agent context; a resumed lineage-bearing session can still be a runtime root.
 

packages/core/agent/src/index.ts

@@ -440,6 +440,18 @@ export class AgentRegistry extends Service {
     return this.store.get(id)?.agent
   }
 
+  /**
+   * Test whether a live agent was created through one exact parent agent's
+   * scoped context. Runtime ownership is independent of durable session
+   * lineage and remains unambiguous when unrelated providers reuse an id.
+   * @param id - the candidate child agent's shared agent/session id.
+   * @param owner - the expected runtime creator agent.
+   * @returns true only while the exact child entry is live under that owner.
+   */
+  isOwnedBy(id: SessionId, owner: Agent): boolean {
+    return this.store.get(id)?.owner === owner
+  }
+
   /**
    * All live agents, in registration order.
    * @returns a fresh array; mutating it does not affect the registry.

packages/core/agent/tests/agent.spec.ts

@@ -72,8 +72,12 @@ describe('AgentRegistry', () => {
 
     expect(ctx.agents.list()).toEqual([root, child])
     expect(ctx.agents.roots()).toEqual([root])
+    expect(ctx.agents.isOwnedBy(child.id, root)).toBe(true)
+    expect(ctx.agents.isOwnedBy(root.id, root)).toBe(false)
+    expect(ctx.agents.isOwnedBy(SessionId('missing'), root)).toBe(false)
 
     detachChild()
+    expect(ctx.agents.isOwnedBy(child.id, root)).toBe(false)
     detachRoot()
   })