feat: add unified_review MCP prompt + sync tool/prompt counts

Adds the unified_review MCP prompt (invoked as /code-review-graph:unified_review)
that drives the unified-review workflow: graph context -> detect_changes ->
score_review -> findings -> dedupe_findings -> generate_report. READ-ONLY:
every finding waits for a manual fix decision. Takes base and tier
(fast/standard/strict) arguments.

- prompts.py: unified_review_prompt + docstring count 5->6
- main.py: @mcp.prompt() registration
- tests/test_prompts.py: TestUnifiedReviewPrompt + preamble test
- docs: LLM-OPTIMIZED-REFERENCE, COMMANDS, CLAUDE.md, INDEX, architecture,
  README + 4 localized READMEs; tool count 30->31 and prompt count 5->6
  synced (historical ROADMAP/FEATURES version notes left unchanged)

README.ja-JP.md

@@ -255,8 +255,8 @@ code-review-graph serve            # MCPサーバーの起動
 | `list_repos_tool` | 登録済みリポジトリの一覧 |
 | `cross_repo_search_tool` | 全登録リポジトリを横断検索 |
 
-**MCPプロンプト**（5つのワークフローテンプレート）：
-`review_changes`, `architecture_map`, `debug_issue`, `onboard_developer`, `pre_merge_check`
+**MCPプロンプト**（6つのワークフローテンプレート）：
+`review_changes`, `architecture_map`, `debug_issue`, `onboard_developer`, `pre_merge_check`, `unified_review`
 
 </details>