Add PR URL copy and OSC52 fallback

[kcosr]

Adds PR URL copy and clipboard fallback support:

• Y copies only the selected pull request URL
• clipboard copying falls back to OSC 52, including tmux passthrough handling, when platform clipboard commands are unavailable or fail
• when OSC 52 is used, the UI reports Sent OSC52 copy... instead of treating it as confirmed native clipboard copy

Note:

• OSC 52 is best-effort; terminal/tmux support is still required for the clipboard to actually receive it.

Validation:

• bun run typecheck
• bun run lint
• bun run format:check
• bun test