Bedrock: forked chat's first turn fails with 400 'nothing available to cache'

Description

Forking a chat and sending the first message fails on Claude-on-Bedrock with:

HTTP 400 — There is nothing available to cache. Please remove the invalid cache point and try again.

applyCaching() in packages/opencode/src/provider/transform.ts always stamps a Bedrock cachePoint, even when the prompt is below Bedrock's minimum cacheable size (~1024 tokens for Claude). A freshly-forked session's first turn is under that minimum, so Bedrock rejects the whole request instead of just running without a cache hit. The Anthropic API ignores an undersized cache_control; Bedrock hard-errors.

Steps to reproduce

1. Use a Claude model via @ai-sdk/amazon-bedrock.
2. Fork a chat (creates a new, short session).
3. Send a short first message.
4. Request fails with HTTP 400 "nothing available to cache".

OpenCode version

1.18.24

Operating System

macOS