bug: clean picker has no viewport — long worktree lists overflow the terminal

[sauravpanda]

Problem

The bonsai clean picker (internal/tui/picker.go) renders every item unconditionally. With more worktrees than terminal rows, bubbletea's inline renderer cannot repaint cleanly: rows scroll out of view, the header/help line disappears, and redraws garble.

Suggested fix

• Add a scrolling viewport to the picker (window of N items that follows the cursor), or switch to bubbles/list.
• Keep the title, help line, and selection summary pinned.

Notes

Only reproducible with many worktrees (roughly > terminal height / 2, since each item renders two lines).

[sauravpanda]

Resolved by #69, which adds a terminal-sized scrolling viewport and keeps picker guidance visible.

[sauravpanda]

Reopening because #69 was merged into the already-landed feature branch rather than main. The picker fix is being resubmitted against main and this issue should close when that PR merges.