Add Codex client attribution

[aezell]

Summary

• add fixed privacy-safe client attribution headers to the hosted Sprites MCP configuration
• attribute requests to Codex via the client-signals agent marker
• document both headers and that attribution is advisory analytics only

Headers

Fly-Client-Agent: codex
Fly-Client-Interactive: false

Fly-Client-Agent carries the attribution. Fly-Client-Interactive is the
instrumentation sentinel that client-signals requires before it reads the
marker at all — with a valid agent marker present, requests classify as agent
traffic regardless of its value. A static plugin config cannot observe whether a
session is attached to a terminal, so it sends a constant rather than a
measurement.

User impact

The Fly backend can distinguish calls initiated through the Codex plugin without
changing authentication, authorization, or tool behavior.

Testing

• validated the exact Codex MCP http_headers configuration with jq
• confirmed http_headers is a supported streamable_http field in Codex's MCP
  config, and that it merges as a transport default header without displacing
  the plugin's OAuth flow
• confirmed header names and the codex marker value against the
  client-signals spec fixtures
• git diff --check

Deployment

No backend deployment is required. The updated plugin configuration takes effect
when the plugin update is distributed.