- --store-credential: after snapshot, writes build-user credentials into
the system keyring using the two-entry scheme (service:meta + service)
that KeyringCredentialStore.get() reads. Works on Windows (Credential
Manager) and Linux (SecretService / file backend).
- --credential-target: keyring target name, default BuildVMGuest (matches
config.guest_cred_target).
- Rewrote module docstring: full prose documentation of both `template
backup` and `template prepare-win` including all 12 provisioning steps.
- 2 new tests covering --store-credential with custom and default targets.
- mypy --strict clean; coverage 90.03%.