fix: address cubic review — broaden error detection, add test coverage

1. isAgentNotFoundError now handles:
   - Plain objects with .message field (not just Error instances)
   - "agent.name"/"undefined" error variants from SDK validation
   - The original "Agent not found" format

2. New tests:
   - agent.name/undefined error variant triggers fallback
   - Plain object errors with .message field trigger fallback
   - "fallback also fails" test now verifies retry was attempted (callCount=2)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/features/background-agent/spawner.ts

@@ -11,8 +11,19 @@ import type { ConcurrencyManager } from "./concurrency"
 export const FALLBACK_AGENT = "general"
 
 export function isAgentNotFoundError(error: unknown): boolean {
-  const message = error instanceof Error ? error.message : String(error)
-  return message.includes("Agent not found")
+  const message =
+    typeof error === "string"
+      ? error
+      : error instanceof Error
+        ? error.message
+        : typeof error === "object" && error !== null && typeof (error as { message?: unknown }).message === "string"
+          ? (error as { message: string }).message
+          : String(error)
+  return (
+    message.includes("Agent not found") ||
+    message.includes("agent.name") ||
+    (message.includes("agent") && message.includes("undefined"))
+  )
 }
 
 export function buildFallbackBody(