Make standby setup fail safely

[gauravtiwari]

Fixes the production-shaped failure modes found while adding a third standby:

• resolve the pgBackRest restore command before entering SSHKit remote scope
• validate required credentials before the first mutation
• scope rollback to only the requested standby instead of visiting existing cluster nodes

No version or gem release changes are included.

Verification:

• 357 tests, 1,058 assertions
• focused RuboCop clean
• existing GCP standby repaired and verified active with zero replay lag