[FEATURE] Welcome/What's New banner dismisses too quickly — add manual dismiss or configurable duration

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

The "What's New" / welcome banner that appears on startup auto-dismisses after a few seconds. This is too fast to read the content, especially for users who want to stay informed about new features and changes.

The banner disappears before you can finish reading it, and there's no way to bring it back (short of running /changelog manually, which shows the full changelog rather than the curated highlights).

Proposed Solution

One or more of:

1. Manual dismiss — require a keypress (e.g., Escape or Enter) to dismiss the banner, instead of auto-timeout
2. Configurable duration — add a setting like whatsNewDuration (in seconds, or 0 for manual dismiss)
3. Longer default — increase the auto-dismiss timeout to give users enough time to read the content

Alternative Solutions

• Users can run /changelog after startup, but this shows the full changelog rather than the curated "what's new" highlights
• A /whatsnew command that re-displays the startup banner on demand

Priority

Low - Nice to have

Feature Category

CLI commands and flags

Use Case Example

Starting a new Claude Code session after an update. The welcome banner flashes "What's New" content but disappears in ~3 seconds — not enough time to read multi-line release highlights.

Additional Context

Related but distinct from #13881 (statusLine replacing native tips) and #15355 (preserveFullHeader with custom statusLine). This is specifically about the auto-dismiss timing of the startup banner.