Allow failed agent turns to be forked (#2063)

* fix(chat): allow failed turns to be forked

Failed turns can end in Paseo-generated timeline items without provider
message IDs. Anchor forks to canonical timeline positions while retaining
legacy boundaries for compatibility.

Place the fork summary before the new prompt and delimit it as structured
chat history.

* fix(chat): tighten fork boundary handling

packages/protocol/src/messages.attachments.test.ts

@@ -1,12 +1,55 @@
 import { describe, expect, it } from "vitest";
 
 import {
+  AgentForkContextRequestMessageSchema,
+  AgentForkContextResponseMessageSchema,
   CreateAgentRequestMessageSchema,
   CreatePaseoWorktreeRequestSchema,
   SendAgentMessageRequestSchema,
 } from "./messages.js";
 
 describe("shared messages attachments", () => {
+  it("preserves an optional timeline cursor on fork-context messages", () => {
+    const boundaryCursor = { epoch: "timeline-1", seq: 42 };
+    const request = AgentForkContextRequestMessageSchema.parse({
+      type: "agent.fork_context.request",
+      agentId: "agent-1",
+      requestId: "fork-1",
+      boundaryCursor,
+    });
+    const response = AgentForkContextResponseMessageSchema.parse({
+      type: "agent.fork_context.response",
+      payload: {
+        requestId: "fork-1",
+        agentId: "agent-1",
+        attachment: null,
+        itemCount: 2,
+        boundaryMessageId: null,
+        boundaryCursor,
+        error: null,
+      },
+    });
+
+    expect(request.boundaryCursor).toEqual(boundaryCursor);
+    expect(response.payload.boundaryCursor).toEqual(boundaryCursor);
+  });
+
+  it("accepts a legacy fork-context response without a timeline cursor", () => {
+    expect(
+      AgentForkContextResponseMessageSchema.parse({
+        type: "agent.fork_context.response",
+        payload: {
+          requestId: "fork-1",
+          agentId: "agent-1",
+          attachment: null,
+          itemCount: 2,
+          boundaryMessageId: "assistant-1",
+          error: null,
+        },
+      }).payload.boundaryCursor,
+    ).toBeUndefined();
+  });
+
   it("keeps valid review attachments", () => {
     const parsed = SendAgentMessageRequestSchema.parse({
       type: "send_agent_message_request",