Anthropic prompt caching misses when dynamic user.system is merged into the static prefix #20110

Summary

Anthropic prompt caching misses when OpenCode merges the session-static system prompt with per-turn user.system content before the request is built.

Problem

packages/opencode/src/session/llm.ts currently joins the static OpenCode system prefix with dynamic caller-provided user.system text into one system message. For Anthropic-style caching, that makes the cached system prefix change on every follow-up turn.

Expected behavior

The static OpenCode prefix should stay in its own system message so it can remain cache-stable across turns, while dynamic user.system content can vary independently.

Scope

A minimal fix would keep the static prefix as the first system message, move dynamic user.system content into a second system message, and keep plugin-added system fragments collapsed back into the trailing slot.