[Feature] Add OpenCode Go session header support

The Goal / Use Case

OpenCode Go requests need an x-opencode-session header associated with the active conversation. This requirement applies only to OpenCode Go, not standard OpenCode Zen.

PicoClaw already tracks a session ID, but its OpenAI-compatible provider cannot currently map that value to a provider-specific request header.

Proposed Solution

Add a dedicated opencode-go provider built on the existing OpenAI-compatible transport.

Add a generic, opt-in provider setting that maps PicoClaw's existing tracked session ID to a named request header. Configure the opencode-go provider to use x-opencode-session.

Potential Implementation (Optional)

• Add an optional generic provider setting whose value is a request-header name.
• When configured, set that header to PicoClaw's tracked session ID.
• Configure the opencode-go provider with x-opencode-session.
• Reuse the existing OpenAI-compatible transport for Chat Completions.

Impact & Roadmap Alignment

• This is a Core Feature
• This is a Nice-to-Have / Enhancement
• This aligns with the current Roadmap

The change is opt-in and should not affect other providers or standard OpenCode Zen.

Alternatives Considered

• Static custom_headers: A fixed value cannot map requests to PicoClaw's tracked session for each conversation.
• Send the header for all OpenAI-compatible providers: Rejected because x-opencode-session is specific to OpenCode Go.
• Add the header to standard OpenCode Zen: Rejected because the requirement does not apply to standard OpenCode Zen.

Additional Context

• OpenCode Go documentation: https://opencode.ai/docs/go/
• OpenCode Go announcement: https://x.com/opencode/status/2095410501400289576
• Provider configuration: docs/guides/providers.md
• OpenAI-compatible transport: pkg/providers/openai_compat/provider.go
• Session manager: pkg/session/manager.go