opencode-go: requests have no x-opencode-session and no User-Agent, and the provider refuses them (400 MissingSessionID) #4

Summary

A run on opencode-go/deepseek-v4.1-flash gets this answer from the provider:

400 {"type":"MissingSessionID","message":"Request is missing x-opencode-session and cannot be routed efficiently. Please see https://opencode.ai/docs/go/#where-can-i-use-it"}

The credential resolves and broca admits the run. The provider refuses the first request after about 1 s, with 0 tokens. Thus each opencode-go run through broca fails.

Version: v0.3.249 (6368883). The related files are the same on master.

Provider requirement

The OpenCode Go documentation (https://opencode.ai/docs/go/#where-can-i-use-it) says that a client should:

1. Identify itself with its own user agent, such as my-coding-agent/1.0, not with a generic SDK or HTTP-library name.
2. Send a stable session ID in x-opencode-session for each conversation.

What broca sends

• The catalog overlay sends opencode-go through the openai_chat family (crates/broca-catalog/data/overlay.json:851-856).
• The openai_chat endpoint sets only content-type, accept, and the auth header (crates/broca-provider/src/families/openai_chat.rs:64-95).
• The provider HTTP client does not set a User-Agent (crates/broca-provider/src/transport.rs:592).
• The request context has a per-session address (transport.rs:211, :461), but the openai_chat family does not send it as a header.

Proposed fix

For the opencode-go provider:

• Send x-opencode-session with a value that is stable for one broca session, for example the session address.
• Send a broca User-Agent. The Anthropic family already has broca/<version> (BROCA_USER_AGENT, crates/broca-provider/src/families/anthropic_messages.rs:60).