OpenCode Go: add x-opencode-session header (required from Sep 6, 2026)

Summary

OpenCode (opencode.ai) emailed OpenCode Go subscribers that requests missing the x-opencode-session header will start erroring on 2026-09-06. Forge's OpenCode Go integration does not send this header.

From the OpenCode Go docs:

To ensure your account does not get flagged, make sure the tool you're using

1. does not generate abusive traffic
2. properly identifies itself (no broad user agents)
3. includes the x-opencode-session header so we can optimize prompt caching

The email states the Forge user agent is not recognized and asks maintainers to add x-opencode-session (one stable ID per conversation).

Current state

• Confirmed against latest main: no occurrence of x-opencode-session anywhere in the repo.
• crates/forge_repo/src/provider/provider.json (opencode_go) has no headers set, and Provider::custom_headers is a static map — insufficient here since the value must be a stable per-conversation ID.
• Reproduced with forge 2.13.21 logged into opencode_go.

Suggested fix

Attach x-opencode-session: <conversation-id> (e.g. the existing conversation/session UUID) to requests routed to the opencode_go provider (and likely opencode_zen as well — same gateway, https://opencode.ai/zen/go).

A static header won't work; the value should be one stable ID per conversation so OpenCode can optimize prompt caching.

Environment

• forge 2.13.21 (latest), macOS arm64
• provider: opencode_go, model: deepseek-v4-flash