feat: add custom command hooks for timer transitions

[thewatts]

Closes #38

Summary

• Adds three new tmux options: @pomodoro_on_complete_cmd, @pomodoro_on_break_end_cmd, and @pomodoro_on_long_break_end_cmd
• These allow users to run custom commands when a Pomodoro completes, a break ends, or a long break ends
• Commands run in the background so they don't block the timer

Example usage

set -g @pomodoro_on_complete_cmd 'tmux display-popup -E "echo Time for a break!; sleep 3"'
set -g @pomodoro_on_break_end_cmd 'tmux display-message "Break over — back to work!"'

Test plan

• Added tests for run_custom_cmd (executes command, no-ops when empty)
• All existing tests pass
• Manual testing: set short timer with custom commands configured, verify they fire at each transition

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