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).

reviewer-prompt.md

@@ -27,8 +27,28 @@ is a real weakness.
 
 {ARTIFACT}
 #  For mode=plan:        path to plan file, OR full inline plan text
-#  For mode=code:        "Use git status / git diff to find changes."
-#  For mode=code-vs-plan: plan path or inline plan + "use git for code changes"
+#  For mode=code:        "Find the changes via git. Review the UNION of
+#                         three independent scopes — do not stop after
+#                         the first non-empty one:
+#                           (a) unstaged: `git diff`
+#                           (b) staged:   `git diff --cached`
+#                           (c) branch-vs-base: resolve <base> by
+#                               trying `git symbolic-ref
+#                               refs/remotes/origin/HEAD`, then
+#                               `origin/main`, `origin/master`, `main`,
+#                               `master`; then
+#                               `git diff <base>...HEAD` plus
+#                               `git log <base>..HEAD` for commit context.
+#                         Do not skip (c) because (a) or (b) are
+#                         non-empty — the typical PR has commits on the
+#                         branch and an in-progress local tweak on top,
+#                         and the operator is asking about both. If the
+#                         scopes are mixed (e.g., committed changes plus
+#                         unstaged ones), label findings by scope where
+#                         it matters and surface the mixed state in the
+#                         summary."
+#  For mode=code-vs-plan: plan path or inline plan + the same union git
+#                         lookup instruction as mode=code
 
 ## Round