feat: export selected hunk to pi

[benvinegar]

Summary

• add a focused-hunk export path that writes .hunk/pi-selection.json for the local Pi bridge
• wire the p shortcut, menu action, help copy, and footer flash message into the Hunk review shell
• cover the payload serialization and keyboard export flow with tests

Notes

• this PR only covers the Hunk side of the bridge
• the matching Pi extension remains project-local and ignored under .pi/

Testing

• bun run typecheck
• bun test