Allow customizing color/style of AskUserQuestion and permission prompts

Problem

When Claude Code is waiting on me for input — an AskUserQuestion choice, a permission prompt, or any blocking decision — the prompt looks visually similar to the rest of the conversation output. In a long-running session it's easy to miss that Claude has stopped and is waiting on me, especially after stepping away from the terminal.

Proposal

Expose a way to style "needs user action" UI elements differently from informational output. Concretely, any of these would solve it:

1. New theme keys for prompt-style messages, e.g. theme.askUserQuestion.color, theme.permissionPrompt.color, settable in ~/.claude/settings.json.
2. A documented ANSI-prefix setting per prompt type.
3. A new hook event (e.g. PromptShown / AwaitingUserInput) that fires when Claude renders a question, so users can inject their own bell, notification, or terminal escape sequence.

Today there's no built-in mechanism for this — theme only controls overall appearance, and existing hooks (PermissionRequest, Notification, Stop) either don't fire at the right moment or can't influence the prompt's rendering.

Why this matters

Visual differentiation is a low-cost, high-signal accessibility / ergonomics win for anyone running long Claude Code sessions. It also helps screen readers and color-blind-friendly themes treat blocking prompts as a distinct class of output.