X-Session-Id not recognized by LiteLLM / observability tools — should follow x-<vendor>-session-id convention

Problem

OpenCode sends X-Session-Id (and x-session-affinity) with every LLM request so providers
can correlate calls across sessions. I confirmed both headers reach the server — but neither
one is recognized by LiteLLM (the most widely used open-source LLM proxy) for session grouping.

Root cause

LiteLLM auto-detects session IDs from headers following this pattern:

x--session-id e.g., x-claude-code-session-id

"Any header matching the pattern x-<vendor>-session-id (for example, x-claude-code-session-id)
is auto-detected as the session id if neither explicit LiteLLM header is present."
— LiteLLM Request Headers docs

Neither of OpenCode's headers match:

If the header were X-Opencode-Session-Id, LiteLLM would pick it up automatically.
Same for any other proxy or observability tool that follows this convention.

What happens today

Every OpenCode request through a LiteLLM proxy gets logged as a separate, ungrouped session.
This breaks per-session cost tracking, conversation tracing, agent loop debugging, and
session-based budget limits.

Proposed fix

In packages/opencode/src/session/llm/request.ts, add one line after line 198:

            "x-session-affinity": input.sessionID,
            "X-Session-Id": input.sessionID,
+           "X-Opencode-Session-Id": input.sessionID,
            ...(input.parentSessionID ? { "x-parent-session-id": input.parentSessionID } : {}),

This is:

• Additive — X-Session-Id stays untouched, no breakage for anyone relying on it today
• One line — zero risk
• Self-documenting — X-Opencode-Session-Id tells any tool exactly what it is and where it comes from
• Follows RFC 6648 — incorporates the organization name in custom headers

Why this isn't LiteLLM-specific

The x-<vendor>-session-id pattern isn't owned by LiteLLM — it's an emerging convention in
LLM infrastructure. A header named X-Opencode-Session-Id also helps any custom proxy,
gateway, or observability stack identify where session traffic originates from, without
cross-referencing other headers.