feat(walkthrough): guided AI walkthrough for diffs, branches, and PRs (#2572)

A diff is ordered by file path, which is almost never the order in which a
change makes sense. This adds a Walkthrough surface that reorders it: the model
groups related hunks into stops, explains what each group changes about
behavior, and orders the stops so each builds on the last. It explains and
orders; judging code stays with the existing Review action.

Reviews uncommitted work (all, staged, unstaged), a branch against its base, or
a pull request. Generation is always user-initiated — nothing runs on a timer,
on a file change, or as a side effect of opening a panel.

Invariants worth preserving:

- Hunk identity is derived on the server and only there. Ids are content
  hashes, so an anchor that no longer resolves is proof the code it described
  changed, and staleness needs no heuristics. The client matches ids to ids and
  never recomputes them; two implementations would have to agree forever.
- The digest is never truncated. A diff that does not fit the model's context
  is refused with an actionable reason, because a walkthrough written against
  half a diff reads as confident and is wrong.
- Nothing disappears. Lockfiles and other generated output are excluded from
  the model's input by name — never by size — and everything no stop covers is
  listed at the end, so "have I seen all of it" stays answerable.
- Cost is explicit. Results are content-addressed, so returning the working
  tree to an earlier state costs nothing; generation outlives its request, so a
  refresh detaches the client rather than discarding paid-for work, and only an
  explicit cancel stops it.

Supporting changes to shared modules:

- git: expose the existing getRangeDiff as GET /api/git
  listUntrackedPaths and getUntrackedDiffs. The latter resolve the repository
  once for a batch instead of per file, taking a panel
  ~340ms on an 80-file working tree.
- small-model: structured output across four wire forma
  and abort signal, and an onOverflow policy so an oversized prompt fails
  loudly instead of being silently clipped. A provider
  remembered so the prompt-side fallback goes first next time.
- models.dev metadata: surface structured_output as tri
  false blocks a model, a missing field does not, because the catalog omits it
  for roughly half of all models.

Desktop and tablet only: VS Code serves Git through its
these routes, and the mobile shell does not consume the surface registry.

Docs: packages/docs walkthrough page in English and all eight locales.

packages/ui/src/lib/i18n/messages/ko.settings.ts

@@ -1520,6 +1520,10 @@ export const settingsDict = {
   'settings.openchamber.defaults.smallModel.useDefault': '기본 소형 모델 사용',
   'settings.openchamber.defaults.smallModel.useDefaultAria': '기본 소형 모델 사용',
   'settings.openchamber.defaults.smallModel.overrideModel': '재정의 모델',
+  'settings.openchamber.defaults.walkthroughModel.title': '변경 워크스루 모델',
+  'settings.openchamber.defaults.walkthroughModel.description': '변경 사항의 AI 리뷰에는 구조화된 출력과 전체 diff를 담을 여유가 필요한데, 저렴한 스몰 모델로는 부족할 때가 많습니다. 카탈로그가 구조화된 출력을 지원하지 않는다고 알리는 모델은 이 목록에서 숨겨집니다. 선택하지 않으면 스몰 모델이 사용됩니다.',
+  'settings.openchamber.defaults.walkthroughModel.overrideModel': '워크스루 모델',
+  'settings.openchamber.defaults.walkthroughModel.usesSmallModel': '스몰 모델',
   'settings.openchamber.defaults.field.showDeletionDialog': '삭제 확인 대화상자 표시',
   'settings.openchamber.defaults.field.openFilesPreviewAria': '미리보기 가능한 파일을 미리보기 모드로 열기',
   'settings.openchamber.defaults.field.openFilesPreview': '미리보기 가능한 파일을 미리보기 모드로 열기',