Polish PR picker UI

[RestartDK]

Summary

• redesign PR picker into header, scrollable list, details, and controls sections
• show changed lines, additions, deletions, and changed file count
• keep selected PR URL and agent context visible in details
• make open/copy URL work better over SSH by using OSC 52 terminal clipboard fallback
• bump Scatterer to v0.1.5

Validation

• cargo check
• SDKROOT=$(xcrun --show-sdk-path) RUSTFLAGS="-L native=$(xcrun --show-sdk-path)/usr/lib" cargo test
• gh pr view 7 --json additions,deletions,changedFiles to verify GitHub fields