Drop the old focus-picker popover (Working tree / Your stack / Other PRs /
Branches) in favour of a narrower, more focused UI inspired by stacked-PR
review tools that only handle in-stack navigation.

Frontend
- New stack breadcrumb in the header: main > #294 > #295 (reviewing) >
  #296, rendered inline (not a popover). Default-branch click flips
  diff_scope to full_stack on the current focus; other entries swap focus
  via /api/focus. Visibility uses stack.length > 1 (not is_stacked) so
  local-only stacks and Sapling-style commit chains also get the
  breadcrumb. Long stacks collapse the middle into an ellipsis.
- New Working tree pill, visible whenever focus is range in git mode.
- Layer/full-stack toggle now appears whenever default_sha is resolved,
  not only for is_stacked focus.
- Other PRs / Remote branches sections deleted from the frontend; backend
  /api/picker still returns them but the frontend stops consuming them.
- focus-changed SSE handler fixed to parse the wrapper.content payload
  correctly (the old handler relied on page reloads in tests to mask this).

Backend (picker.go)
- detectStack adds tier-3 fallback for naked-commit ancestors on the topic
  chain (commits reachable from HEAD but not the default branch). Uses
  first-line commit subject as the label so Sapling-style git stacks of
  unbranched commits surface in the picker.
- Topic-chain filter prevents long-lived feature branches from drowning
  the breadcrumb in default-branch ancestors.

CLI
- Help text now lists --pr <num|url> and --range <baseSHA>..<headSHA> as
  the entry points into range mode.

Tests
- Rewrote focus-picker.rangemode.spec.ts and focus-picker-wt-entry for
  the breadcrumb. Added breadcrumb-current-marker,
  breadcrumb-default-branch, breadcrumb-ellipsis, breadcrumb-local-stack,
  breadcrumb-no-pr specs.
- Updated focus-switch.rangemode for the working-tree pill.
- loading.filemode + nogit.nogit assert the breadcrumb and pill stay
  hidden in non-VCS modes.
- range-loading header-label test now checks the breadcrumb reviewing
  marker instead of the removed #focusPickerLabel.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>