fix: replace raw echo with output helpers in general/ssh-key-mgr

[github-actions]

Summary

• Replaces the non-standard die() with the canonical implementation (TTY-aware coloured output to stderr, exits non-zero)
• Adds success() and info() helpers alongside die()
• Replaces all user-facing echo calls with info (progress messages, audit output, warnings)
• Converts the help block from multiple echo calls to a single cat <<EOF heredoc
• Fixes pre-existing shellcheck SC2236 warnings: [ ! -z ... ] → [ -n ... ]
• Preserves echo calls that write to the SSH config file and those used as function return values via command substitution

Closes #637

[credfeto]

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[dnyw4l3n13]

All CI checks have passed. The implementation has been reviewed and is complete:

• die(), success(), and info() helpers match the canonical implementations from shell-scripts.examples.md
• All user-facing echo calls replaced with appropriate helpers (info for progress/audit output)
• Help block converted to a single cat <<EOF heredoc
• Pre-existing shellcheck SC2236 warnings fixed ([ ! -z ... ] → [ -n ... ])
• echo calls writing to the SSH config file and those used as function return values preserved correctly

PR is ready for review.

[credfeto]

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