Detect which AI coding agent is invoking gh by checking well-known
environment variables and include the agent name in the User-Agent
header sent to GitHub APIs.
Supported agents: Codex, Gemini CLI, Copilot CLI, OpenCode,
Claude Code, and Amp. Generic AI_AGENT env var is also supported
with validation to prevent header injection.
Fixesgithub/cli#1111
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
