fix(orbit): say Orbit instead of GKG in graph status error copy

Description

The glab orbit command surface is already named Orbit everywhere, but the HTTP 503 message from glab orbit remote graph-status still called the backend "GKG", the old internal name. Anyone hitting that error saw a service name that appears nowhere else in the CLI.

This renames the wording to Orbit in the error text, the comment above it, and the test name. Display strings only, no behavior change.

Related Issues

Trivial copy change, no issue.

How has this been tested?

go test ./internal/commands/orbit/... passes, including the renamed TestGraphStatus_OrbitServiceUnavailable.