test: add tmux-based picker snapshot regression checks

[neilberkman]

Summary

• add deterministic golden snapshot test for Bubble Tea picker rendering
• add tmux capture-pane script to validate terminal-rendered snapshot against golden output
• wire snapshot regression step into PR CI workflow

Details

• New test file: cmd/clippy/picker_snapshot_test.go
  • TestPickerSnapshotGolden compares normalized picker view output to cmd/clippy/testdata/picker_snapshot.txt
  • TestPickerSnapshotPrint emits snapshot markers for tmux capture flow
• New script: scripts/picker_snapshot_tmux.sh
  • runs snapshot print test in tmux
  • captures pane content
  • diffs against golden snapshot
• PR workflow updates:
  • trigger on snapshot/script changes
  • run tmux snapshot regression step

Validation

• go test ./cmd/clippy -run 'TestPickerModel|TestPickerSnapshotGolden' -count=1
• ./scripts/picker_snapshot_tmux.sh