feat(skill): union-of-scopes детект изменений для code-режима

- Зачем:
  - типичный PR-флоу: ветка ahead of base + (часто) локальный твик поверх. Прежняя логика смотрела только unstaged/staged и пропускала коммиты ветки, давая пустое или неполное ревью.
- Что:
  - Step 1 auto-detect: добавлен третий независимый scope `git diff <base>...HEAD` с резолвом base (`symbolic-ref refs/remotes/origin/HEAD` → `origin/main` → `origin/master` → `main` → `master`).
  - Step 2 dispatch: briefing для `code` и `code-vs-plan` переписан с fallback на union трёх скоупов, явное "do not skip branch-vs-base because working tree is non-empty".
  - reviewer-prompt.md: `{ARTIFACT}` placeholder теперь спеллует все три scope (a)(b)(c) с инструкцией про labeling mixed-state.
  - README.md: таблица "Three modes" и пояснение синхронизированы с union-семантикой.
- Проверка:
  - на ветке feat/cross-platform-receiving-review одновременно непустые `git diff` и `git diff origin/main...HEAD` — финальная формулировка прошла Codex review (3 раунда, VERDICT: APPROVED).

README.md

@@ -73,14 +73,21 @@ runs without operator access conclude unverified.
 
 ### Three modes
 
-| Mode           | What it reviews                    | When to use              |
-|----------------|------------------------------------|--------------------------|
-| `plan`         | Implementation plan                | Before writing code      |
-| `code`         | Git diff (unstaged, staged, branch)| After writing code       |
-| `code-vs-plan` | Code changes against the plan      | Verify implementation    |
+| Mode           | What it reviews                          | When to use              |
+|----------------|------------------------------------------|--------------------------|
+| `plan`         | Implementation plan                      | Before writing code      |
+| `code`         | Working-tree, staged, and branch changes | After writing code       |
+| `code-vs-plan` | Code changes against the plan            | Verify implementation    |
 
 Mode is auto-detected from context, or you can force it with an argument.
 
+For `code` mode, "branch changes" means commits on the current branch
+since it diverged from its base (`main`, `master`, or the remote
+default). The three scopes (unstaged, staged, branch-vs-base) are
+reviewed as a union, not a fallback chain — so the typical PR case
+(committed changes on the branch plus an in-progress local tweak) is
+covered without an explicit flag.
+
 ### Language
 
 The skill detects the operator's session language from recent