dsh-opencode-session-header: OpenCode Go Session Header Injection in DSH

`dsh-opencode-session-header` is a plugin for DeepSeek Harness (DSH) that fixes the `400 MissingSessionID` error returned by the OpenCode Go model when the `x-opencode-session` request header is missing. The plugin intercepts requests and injects the header, resolving the error while preserving the existing routing and caching optimizations. It only applies to the `opencode.ai` domain, supports a runtime toggle, and has no additional dependencies, making it suitable for use with DSH version 0.1.2-rc.1.