Allow users to customize spinner/loading messages

The spinner messages that appear while Claude is thinking (prefixed with +) are fun but hardcoded. It would be great to let users define their own list via settings.

Use case

I have a pull-up bar next to my desk. I'd love the spinner to show fitness prompts like "Do 10 push-ups", "Hold a 30s plank", "Do 5 pull-ups" — turning Claude's thinking time into micro-workout opportunities.

Proposed implementation

A spinnerMessages field in settings.json:

{
  "spinnerMessages": [
    "Do 10 push-ups",
    "Hold a 30s plank",
    "Do 5 pull-ups",
    "Do 15 squats"
  ]
}

When the list is set, Claude Code picks randomly from it instead of the default messages. When unset, the current defaults ("Razzmatazzing", "Vibing", etc.) remain.

This is a small, low-risk change that adds personality and makes the wait time genuinely useful. The status line already supports custom commands — extending the same idea to spinner messages feels natural.