Keep reminder notifications visible until dismissed

[Brams-s]

Summary

• Send fired reminder notifications at critical urgency, which stays on screen until dismissed.
• Document the acknowledgement-required behavior.
• Add a regression test for the timer payload.

Context

Implements the request in #5784.

This is deliberately limited to fired reminders. It does not change the general critical-notification lifetime discussed in #9361; the existing dismissal paths remain available. #10679 independently changes how reminder timers behave across suspend and may need a trivial rebase if it merges first.

Verification

• bash test/shell.d/reminders-test.sh
• bin/omarchy commands --check
• bash -n bin/omarchy-reminder
• git diff --check
• Visual: a critical reminder stayed visible after six seconds and then dismissed normally.