Add checkpoint backend registry and factory

Introduce a backend registry in the checkpoint package: a Factory type, an OpenEnv construction context, Register/build, and a built-in "git" backend. Open now builds its primary store through the registry (defaulting to the git backend) rather than calling NewGitStore directly.

This is the seam Phase 2 needs: a new persistent backend becomes a factory registered under a type name. Production registers only the git backend; test-only backends will register through their own RegisterForTesting helpers so a production binary can never select them. The registry default of git matches today's Open exactly, so there is no behavior change at default config.