[Helper] Support non-root SSH users for cold standby

[pi0n00r]

Contribution Guidelines

• I've read the contribution guidelines and wholeheartedly agree to them

What does this PR include?

Short Description

This PR allows the cold-standby destination to be reached through a non-root
SSH account while preserving root as the default.

It is a clean reimplementation, based directly on current staging, of the
remote-user goal previously raised in #6612 and #6613. It contains no unrelated
history and no watchdog changes.

The helper now:

• accepts REMOTE_SSH_USER, defaulting to root;
• defaults an omitted REMOTE_SSH_PORT to 22;
• uses one SSH command path for the configured user, key, port, and host-key
  policy;
• treats a non-root user as an SSH transport identity and explicitly requires
  root-equivalent sudo -n access during preflight;
• runs destination filesystem, Docker, Compose, and service operations through
  non-interactive sudo for non-root users;
• supplies sudo -n rsync as the remote rsync path when required;
• accepts directory-backed SSH principals through SSH's -l option; and
• keeps the existing root command path and host-key policy defaults.

The tracked launcher remains root-compatible and documents the non-root option.

Affected Containers

• None (helper scripts only)

Did you run tests?

Yes.

What did you test?

• helper-scripts/tests/test_cold_standby_non_root.sh
• Bash syntax for the launcher and cold-standby helper
• ShellCheck at error severity for both upstream scripts
• Full ShellCheck for the new fixture test
• Root and non-root target construction
• Root and non-root remote rsync command selection
• Exact SSH argument construction, including a key path containing spaces
• Non-root privileged command construction and failure propagation
• Static checks that no hard-coded root@... target remains

This candidate has not yet been exercised in a fresh end-to-end standby
migration.

What were the final results? (Awaited, got)

Expected:

• existing root destinations retain their current command path;
• non-root destinations fail preflight unless passwordless sudo is available;
• non-root rsync and privileged destination operations use sudo -n; and
• invalid user/port configuration fails before data transfer.

Got:

• the deterministic test passed;
• Bash syntax checks passed;
• ShellCheck reported no errors in the touched scripts; and
• the new test script passed full ShellCheck.