chore: add unit tests for cmdutil shared utilities

[rbansal42]

The new shared utilities in internal/cmdutil/ (time.go, user.go, output.go, strings.go) have no unit tests. These are pure functions used by 17+ call sites — a regression here cascades everywhere.

Needed tests:

• TimeAgo: zero time, past, future, boundary values (1 min, 1 hour, 1 day, 1 month, 1 year)
• TimeAgoFromString: empty, RFC3339, Bitbucket microsecond format, invalid
• GetUserDisplayName: nil, empty fields, full fields, nickname-only
• TruncateString: short, exact, over limit, with newlines
• PrintJSON: valid struct, error case
• ConfirmPrompt: y, yes, Y, YES, no, empty, EOF
• PrintTableHeader: color enabled vs disabled