[fix] Fix spinner lifecycle in add and merge commands (#88)

[lugassawan]

Summary

• Fix missing s.Stop() before output in add and merge commands, causing result text to appear on the same line as the spinner
• Change progress callback from s.Start(msg) to s.Update(msg) so all progress steps are visible (especially in non-animated/CI mode)
• Add explicit s.Start() before operations to match the correct pattern used in duplicate and restore commands

Closes #88

Test Plan

• make fmt — clean
• make lint — 0 issues
• make test — 19/19 packages pass
• Manual: rimba add test-task shows step-by-step spinner updates and "Created" on a new line