GitHub forge backend for travelagent.
Implements [travelagent_core::forge::ForgeBackend] against the
GitHub REST + GraphQL APIs. Handles both github.com and GitHub
Enterprise; authentication resolves from GITHUB_TOKEN / GH_TOKEN
/ gh auth token in that order.
The public entry point is [GitHubForge], constructed via
GitHubForge::new() (which resolves the token up-front) and
passed to the TUI or MCP crates via the
[travelagent_core::forge::ForgeBackend] trait.