dsh session IDs for opencode

Key features

• A DeepSeek Harness (dsh) pluginwhen you use an opencode model (providers such as opencode / opencode-go that point at the opencode.ai gateway), it makes the actual outgoing HTTP requests carry a session id — the same way the opencode client itself does.
• The opencode gateway branch uses x-opencode-session to carry the session id
• verify.mjs fires a real opencode-go request through the pi-ai bundled with the dsh CLI, asserting that the wire request headers really carry the session id
• this plugin listens on the llm/stream scope, wraps fetch, and adds that header at the wire layer.
• By default it hashes the uuid portion of session- (the session- prefix is excluded) into an alphanumeric nanoid(8) before it goes on the wire.