fix: add 'usage limit' to rate limit detection patterns

OpenAI/ChatGPT returns "You have hit your ChatGPT usage limit (plus plan)"
when users exceed their plan quota. This error wasn't being recognized as a
rate limit, so fallback to alternative models wasn't triggering.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/agents/pi-embedded-helpers.ts

@@ -343,6 +343,7 @@ const ERROR_PATTERNS = {
     "resource has been exhausted",
     "quota exceeded",
     "resource_exhausted",
+    "usage limit",
   ],
   timeout: [
     "timeout",