prompt-caching 3

Per-conversation x-opencode-session header injection for OpenCode Go in DeepSeek Harness (DSH). Fixes 400 MissingSessionID, keeps prompt-cache routing optimal, scoped strictly to opencode.ai, runtime-switchable without restart.

DeepSeek Harness plugin: DietrichGebert/ponytail lazy senior mode & 7-rung ladder port, 6 skills with bilingual descriptions & per-skill toggles, zero tools, zero-cache-miss | DeepSeek Harness 插件：DietrichGebert/ponytail 懒人 senior 模式与七阶梯子完美移植，6 个技能描述双语自由切换

Keeps long coding-agent sessions lean: wrap a stretch of work in a named task, and when it ends, fold that whole span into one short titled summary. The conversation stays readable, context costs stay low, and any fold's original content can be read back