feat(todo-dock): sweep the working step and drop per-row chrome

[Tioit-Wang]

Why

The expanded session checklist read as a stack of bordered status squares, so
the list competed with the task text for attention and the running step was no
easier to find than any other row.

What changed

• Plain-text rows. The per-row status mark becomes a bare glyph instead of
  a bordered 16px box. Completed strike-through, cancelled muting, and the
  in-progress weight/colour are unchanged.
• One sweep, on the working step's own text. A light band is painted into
  the text with -webkit-background-clip: text, so no row changes shape, gains
  a fill, or reserves space for it. This is now the dock's only motion.
• Exactly one shimmer at a time. While collapsed it rides the header's
  current-item line (that is where the running step is shown); while expanded it
  moves to the row and the header goes quiet. The list stays mounted behind the
  disclosure, so without that gate the same text would sweep twice.
• Reduced motion. A prefers-reduced-motion user gets flat primary ink
  instead of a band frozen mid-sweep, rather than animation: none alone.
• The bordered panel, the disclosure behaviour, the session-scoped recovery,
  and the i18n keys are all unchanged.

Band geometry is what makes the loop hold: a 200% image sliding 100% → 0%
keeps the visible window inside the image at every frame, so it neither drops
out nor restarts visibly. Both gradient stops are existing --ds-text-* tokens,
so each theme keeps its own contrast and a contributed theme can still move
them.

Verification

• tsc -p apps/desktop/tsconfig.json --noEmit — 0 errors.
• apps/desktop/test/todo-dock-rendering.test.mjs, todo-events.test.mjs,
  todo-recovery.test.mjs — 8/8 pass. The rendering test asserts exactly one
  shimmer in the collapsed markup, that it sits on the header's current-item
  line, and that the marker keeps no border/radius; the CSS test asserts the
  clip, the 200% geometry, the keyframe, and the reduced-motion fallback.
• scripts/check-style-tokens.mjs, style-surface-tokens on composer.css,
  check-architecture.mjs, check-agent-policy-sync.mjs, biome lint,
  docs/scripts/check-locales.mjs, docs/scripts/check-docs.mjs — all pass.
• The full apps/desktop suite still reports its pre-existing platform failures
  (macOS codesign/stapler, remote-host supervisor, Windows path fixtures); the
  same files fail identically on a clean origin/main tree, and none of them
  touch todo.

Specs synced: docs/spec/04-ux/08-component-spec.md §11.3a (and its zh-CN
mirror), docs/spec/06-delivery/04-e2e-test-plan.md (and mirror), ADR 0312.