fix(cumple): use display name (nickname) instead of real username

[gmq]

Summary

• The birthday script was storing and displaying msg.message.user.name (the real Slack username) instead of the user's display name/nickname
• Added a getDisplayName helper that reads user.slack.profile.display_name with a fallback to user.name, following the same pattern used in poll.js
• The brain key still uses the real username for stable lookups; only the displayed user field uses the nickname

Test plan

• Run huemul cumple set DD/MM and confirm the confirmation message and subsequent hoy/mes/lista outputs show the nickname, not the real username
• Confirm huemul cumple delete still removes the correct entry
• Existing unit tests pass (fallback to .name applies in test environments where user.slack is undefined)

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