Unify the ls/ui tree renderer with the shared stack primitive

sc submit and sc restack now render their stack through a shared primitive in
src/shortcake/_stack_view.py (build_layout + StackRenderer). _tree.py —
used by sc ls and the web UI — still has its own renderer, so there are two
tree layouts in the codebase.

Folding _tree.py into the primitive turned out not to be a mechanical
extraction, which is why it was left out of the original change.

Why it's blocked

The two renderers use genuinely different visual models:

ls today:

│ │ ◯ add-world-module
│ │ │ Add world module
│ │ │
◯─┴─┘ add-hello-module
│     Add hello module
│
◯ main
  Initial commit

submit today:

  ● add-github-repository-analysis-service            #4672
  │
  ├─● analyze-repositories-in-github-repository-list  #4673
  │ │
  │ ● expose-github-repository-analysis-api           #4674
  │
  └─● 2026-07-24-add-github-rate-limit-aware-client   #4675

Sharing one layout means picking one of these and changing the other. That's a
UX decision about how ls should look, not a refactor — and ui.py builds on
the same StackTree, so the web UI changes with it.

Options

1. ls adopts indented arms. One layout everywhere. Changes ls output for
   any forked stack, plus the web UI, tests/test_tree.py and
   e2e/docs/21-ls.md. Parallel columns arguably show sibling stacks better,
   so this is a real loss for wide stacks.
2. submit adopts parallel columns. Keeps ls as-is, but the columns are
   hard to reconcile with a status column and with viewport windowing, which
   assumes one row per branch.
3. Share only the tree walk (parent → children, ordering, fork
   detection) and keep two renderers on top. Smaller win, no visual change.
4. Leave both. The duplication is ~80 lines and neither renderer changes
   often.

Option 3 is probably the best value if ls should keep its current look.

Notes

• Direction alone could be a parameter of build_layout; the fork model is the
  part that genuinely conflicts.
• Whatever is chosen, _tree.py's label content (commit subject, worktree
  paths, ⟳ needs restack, PR draft/merged/closed state) is orthogonal and can
  stay where it is.

Read src/shortcake/_tree.py and src/shortcake/_stack_view.py, then inspect the ls and ui entry points and the existing tests in tests/test_tree.py and e2e/docs/21-ls.md. Compare the four listed options with maintainers, especially the fork layout, status column, and web UI impact. Done means the chosen sharing or rendering strategy is implemented with the affected output and tests updated.