fix: treat Anthropic long context billing error as context overflow

Anthropic returns "Extra usage is required for long context requests"
when the prompt exceeds the billing tier's context limit. This was
incorrectly classified as a retryable API error, causing infinite
retries (observed up to attempt #7 with 2m delays).

Classify it as a context overflow so compaction kicks in and reduces
the prompt to fit within the standard context window.

packages/opencode/src/provider/error.ts

@@ -8,6 +8,7 @@ export namespace ProviderError {
   // https://github.com/badlogic/pi-mono/blob/main/packages/ai/src/utils/overflow.ts
   const OVERFLOW_PATTERNS = [
     /prompt is too long/i, // Anthropic
+    /extra usage is required for long context/i, // Anthropic (billing tier context limit)
     /input is too long for requested model/i, // Amazon Bedrock
     /exceeds the context window/i, // OpenAI (Completions + Responses API message text)
     /input token count.*exceeds the maximum/i, // Google (Gemini)