fix: sync picker hint + worktree list perf and output leak

[mswdev]

Summary

Three bugs across two commands — kept in one PR per request.

1. ckipper account sync interactive wizard exited silently

gum choose --no-limit exits 0 with empty stdout when ENTER is pressed without first SPACE-toggling items. The single-select source picker right before it accepts ENTER on its own, so muscle memory made this trivially easy to do — and the wizard returned 1 silently in the targets case and printed a bare No types selected. in the types case, leaving users to think the picker was broken.

Both empty-selection paths now print the same SPACE/ENTER hint to stderr before returning 1.

2. ckipper worktree list was slow — ≈25× faster

find recursed unbounded under $CKIPPER_WORKTREES_DIR with only */node_modules/* excluded, so heavy build/cache trees (dist, .next, target, __pycache__, etc.) were walked in full. On a 6 GB / 380k-file worktrees tree the scan took ~700 ms. Pruning the known-heavy directory names brings it to ~30 ms. Bounding by depth was not viable because branches contain slashes (e.g. feature/OGD-320-…).

# before
$ time ckipper worktree list
...real 0m0.726s

# after
$ time ckipper worktree list
...real 0m0.033s

3. ckipper worktree list leaked spurious branch='…' lines

local branch (no =value) inside the per-iteration loop body behaves like typeset -p branch once the variable already carries a value from a prior iteration, leaking literal lines under each bullet. Hoisted the loop locals above the while, matching the existing precedent for the same hazard in lib/account/sync/preview.zsh.

Test plan

• bats lib/account/sync/dispatcher_test.bats — 19/19 pass (incl. 2 new regressions for the picker hint)
• bats lib/worktree/worktree_test.bats — 15/15 pass (incl. 3 new regressions for pruning, slash-branches, leak)
• make test-unit — 504/504 shell tests pass
• make lint-shell lint-zsh lint-fmt lint-merge-guards — all clean
• Manual: ckipper account sync shows hint when ENTER pressed without SPACE-selecting
• Manual: ckipper worktree list is visibly instant and shows no branch= lines