feat(agent): add isContextOverflowError utility

Detects context overflow 400 errors from various LLM providers
(prompt too long, context length exceeded, request too large, etc.)
for use in auto-compaction recovery.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/core/src/agent/errors.ts

@@ -0,0 +1,21 @@
+/**
+ * Error classification utilities for agent error handling.
+ */
+
+/**
+ * Check if an error is a context overflow / "prompt too long" error from any LLM provider.
+ *
+ * These errors indicate the request exceeded the model's context window and should
+ * trigger auto-compaction rather than auth profile rotation.
+ */
+export function isContextOverflowError(error: unknown): boolean {
+  const msg = (error instanceof Error ? error.message : String(error)).toLowerCase();
+  return (
+    msg.includes("prompt is too long") ||
+    msg.includes("context length exceeded") ||
+    msg.includes("maximum context length") ||
+    msg.includes("request_too_large") ||
+    msg.includes("request size exceeds") ||
+    (msg.includes("413") && msg.includes("too large"))
+  );
+}