Make the clean picker self-explanatory

[sauravpanda]

Summary

• keep the highlighted worktree visible in a terminal-sized scrolling viewport
• make movement, selection, bulk selection, paging, review, and quit controls explicit
• make protected rows view-only unless --force is active and keep bulk selection safe-only
• add a separate review screen before any deletion is confirmed
• return a useful command-line error when interactive mode is launched without a TTY

Why

The picker rendered every worktree at once. With a laptop-wide list of 110 worktrees, most rows and the active cursor fell outside the visible terminal, making the command appear non-interactive. Selection state and the effect of Enter were also too easy to miss.

Impact

Users now see a bounded list with remaining-row counts, a visible cursor, a live selected count, and direct instructions such as “Space to select” and “Enter to review.” Enter no longer exits directly into deletion; it opens an explicit confirmation screen first.

Validation

• go build ./...
• go vet ./...
• go test ./... -count=1
• go test -race ./... -count=1
• real PTY smoke test against 110 Claude worktrees; exited with q and made no changes

Dependency

This is a stacked PR targeting the cleanup branch from #68. After #68 merges, this PR can be retargeted to main.