Force gh-aw extension upgrade resolution to github.com in mixed GHE contexts

[Copilot]

In GHE-authenticated environments, gh aw could report extension state as current while still running an older gh-aw version because upgrade/release resolution followed the active enterprise host context. This change makes extension update resolution deterministic against the canonical upstream source and surfaces host context in user-facing output.

• Canonical host resolution for extension operations

  • Routed all self-upgrade subprocesses (gh extension upgrade/remove/install) through a single helper that enforces GH_HOST=github.com.
  • Prevents false “already up to date” outcomes caused by enterprise-host lookup when the extension origin is github/gh-aw.

• Canonical host resolution for release discovery

  • Configured the go-gh REST client used by getLatestRelease(...) with Host: github.com.
  • Ensures latest-version checks are evaluated against upstream releases, not ambient auth host.

• Clear host context in upgrade UX

  • Added explicit source-host context in upgrade messaging (check/up-to-date/upgrade paths).
  • Reduced ambiguity during mixed-host troubleshooting.

• Targeted test coverage for env override behavior

  • Added/extended unit tests for host env override semantics and command construction (withEnvOverride, githubExtensionCommand).

func githubExtensionCommand(args ...string) *exec.Cmd {
	cmd := exec.Command("gh", args...)
	cmd.Env = withEnvOverride(os.Environ(), "GH_HOST", "github.com")
	return cmd
}