bug: opencode go as custom provider is not working, require session id to be passed as header

## Issue

When using OpenCode go as custom provider, it return this error:
```
Error from provider (Console Go): Request is missing x-opencode-session and cannot be routed efficiently. Please see https://opencode.ai/docs/go/#where-can-i-use-it
```

The docs says:

> Your client should:
> Send typical coding agent traffic
> Identify itself with its own user agent, such as my-coding-agent/1.0, rather than a generic SDK or HTTP-library name.
> Send a stable session ID in x-opencode-session for each conversation so we can optimize routing and prompt caching.

reference: https://opencode.ai/docs/go/#where-can-i-use-it

## Suggestion

Add ability to set custom header in custom provider and set it as literal value or get it from predefined variable (such as session id for this case)