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).

SKILL.md

@@ -56,9 +56,20 @@ message "Plan mode is active" → mode = `plan`, skip git.
 
 **3. Auto-detect** (no explicit argument, not in Plan Mode):
 
-1. Check for code changes (any non-empty output means changes exist):
+1. Check for code changes — there are **three independent scopes**, and
+   any non-empty output from any of them means code changes exist:
    - `git diff --name-only` — unstaged
    - `git diff --cached --name-only` — staged
+   - `git diff --name-only <base>...HEAD` — committed on the current
+     branch since it diverged from its base. Resolve `<base>` by trying
+     in order: `git symbolic-ref refs/remotes/origin/HEAD` (canonical
+     remote default), then `origin/main`, `origin/master`, `main`,
+     `master`. If none resolve, skip this check.
+
+   The three scopes are independent, not a fallback chain. The typical
+   PR has commits on the branch *and* often an in-progress local tweak
+   on top; the briefing in Step 2 reviews the union, not just the first
+   non-empty one.
 2. Check if a **structured plan artifact** exists in the current
    conversation context. This means one of:
    - The user pasted or referenced a markdown plan document with
@@ -112,8 +123,16 @@ commands, and reads files on its own. **Do not pre-collect diffs or file
 lists for it.** Pass mode-specific context only:
 
 - For `plan` — path to plan file, or inline plan text
-- For `code` — instruct it to use git status / git diff
-- For `code-vs-plan` — pass the plan and instruct git lookup for changes
+- For `code` — instruct the reviewer to review the **union of three
+  scopes**: unstaged (`git diff`), staged (`git diff --cached`), and
+  committed since base (`git diff <base>...HEAD`, base resolved the
+  same way as in Step 1). Branch-vs-base is *not* a fallback — do not
+  skip it just because the working tree is non-empty. The typical PR
+  has both. When scopes are mixed, the reviewer should label findings
+  by scope where it matters and surface the mixed-state to the
+  operator.
+- For `code-vs-plan` — pass the plan and instruct the same union git
+  lookup as `code` (all three scopes, not a fallback chain)
 
 Fill the placeholders in `reviewer-prompt.md` and pass the result as the
 subagent's prompt. The template includes operating stance, finding bar,