fix(continuation): mark fallback resumes synthetic

src/plugin/event.model-fallback-pin-agent.test.ts

@@ -18,14 +18,26 @@ function setupConnectedProviderCacheMocks(): void {
 type PromptBody = {
   path: { id: string }
   body: {
-    parts: Array<{ type: "text"; text: string }>
+    parts: Array<{
+      type: "text"
+      text: string
+      synthetic?: boolean
+      metadata?: Record<string, unknown>
+    }>
     agent?: string
     model?: { providerID: string; modelID: string }
     variant?: string
+    noReply?: boolean
   }
   query: { directory: string }
 }
 
+function expectSyntheticContinuation(body: PromptBody["body"]): void {
+  expect(body.noReply).toBeUndefined()
+  expect(body.parts[0]?.synthetic).toBe(true)
+  expect(body.parts[0]?.metadata?.compaction_continue).toBe(true)
+}
+
 describe("createEventHandler - model-fallback auto-continuation pins agent/model/variant", () => {
   const createHandler = (args?: {
     hooks?: any
@@ -127,6 +139,7 @@ describe("createEventHandler - model-fallback auto-continuation pins agent/model
       providerID: "anthropic",
       modelID: "claude-opus-4-7",
     })
+    expectSyntheticContinuation(body)
   })
 
   test("pins agent/model on promptAsync body when continuing after session.error fallback", async () => {
@@ -167,6 +180,7 @@ describe("createEventHandler - model-fallback auto-continuation pins agent/model
       providerID: "anthropic",
       modelID: "claude-opus-4-7",
     })
+    expectSyntheticContinuation(body)
   })
 
   test("pins agent/model on fallback prompt() body when promptAsync is not available (session.status)", async () => {
@@ -223,6 +237,7 @@ describe("createEventHandler - model-fallback auto-continuation pins agent/model
       providerID: "anthropic",
       modelID: "claude-opus-4-7",
     })
+    expectSyntheticContinuation(body)
   })
 
   test("pins variant from agent config when present", async () => {
@@ -268,5 +283,6 @@ describe("createEventHandler - model-fallback auto-continuation pins agent/model
     expect(promptAsyncBodies.length).toBe(1)
     const body = promptAsyncBodies[0]!.body
     expect(body.variant).toBe("thinking")
+    expectSyntheticContinuation(body)
   })
 })