refactor(docs): normalize documentation naming to lowercase for PEP8 compliance (#434)

* refactor(docs): rename directories to lowercase for PEP8 compliance

- Developer-Guide → developer-guide
- Getting-Started → getting-started
- Reference → reference
- Templates → templates
- User-Guide → user-guide
- User-Guide-jp → user-guide-jp
- User-Guide-kr → user-guide-kr
- User-Guide-zh → user-guide-zh

This change aligns with Python PEP8 package naming conventions.
All 43 files affected.

* refactor: rename root documentation files to lowercase

- CHANGELOG.md → changelog.md
- CODE_OF_CONDUCT.md → code_of_conduct.md
- CONTRIBUTING.md → contributing.md
- SECURITY.md → security.md

Aligns with Python package naming conventions (PEP8).
README files remain uppercase as per convention.

* refactor: move documentation files to docs/ for cleaner root

Moved OSS standard files to docs/:
- CHANGELOG.md → docs/CHANGELOG.md
- CODE_OF_CONDUCT.md → docs/CODE_OF_CONDUCT.md
- CONTRIBUTING.md → docs/CONTRIBUTING.md
- SECURITY.md → docs/SECURITY.md

Root now contains only essential files:
✓ README files (表紙: en, ja, kr, zh)
✓ LICENSE (法的要件)
✓ Build configs (pyproject.toml, setup.py, MANIFEST.in)
✓ VERSION

Rationale:
Cleaner root structure following modern Python project conventions.
All detailed documentation consolidated in docs/ directory.

* refactor: update documentation links after restructure

Auto-updated internal documentation links to reflect new structure:
- docs/ subdirectories now lowercase (PEP8)
- Root files moved to docs/
- All cross-references updated

This commit includes linter-generated link updates.

* chore(docs): keep OSS-standard uppercase root files (CHANGELOG, CODE_OF_CONDUCT, CONTRIBUTING, SECURITY)

* chore(docs): remove duplicated PR docs from repo root (moved under docs)

* docs: rename pm-agent-implementation-status.md -> PM_AGENT.md for clarity

* docs: update links to PM_AGENT.md after rename

---------

Co-authored-by: kazuki <kazuki@kazukinoMacBook-Air.local>

README-ja.md

@@ -275,55 +275,55 @@ pip install --break-system-packages SuperClaude
 <tr>
 <td valign="top">
 
-- 📝 [**クイックスタートガイド**](docs/Getting-Started/quick-start.md)  
+- 📝 [**クイックスタートガイド**](docs/getting-started/quick-start.md)  
   *すぐに開始*
 
-- 💾 [**インストールガイド**](docs/Getting-Started/installation.md)  
+- 💾 [**インストールガイド**](docs/getting-started/installation.md)  
   *詳細なセットアップ手順*
 
 </td>
 <td valign="top">
 
-- 🎯 [**コマンドリファレンス**](docs/User-Guide-jp/commands.md)  
+- 🎯 [**コマンドリファレンス**](docs/user-guide-jp/commands.md)  
   *全21のスラッシュコマンド*
 
-- 🤖 [**エージェントガイド**](docs/User-Guide-jp/agents.md)  
+- 🤖 [**エージェントガイド**](docs/user-guide-jp/agents.md)  
   *14の専門エージェント*
 
-- 🎨 [**動作モード**](docs/User-Guide-jp/modes.md)  
+- 🎨 [**動作モード**](docs/user-guide-jp/modes.md)  
   *5つの適応モード*
 
-- 🚩 [**フラグガイド**](docs/User-Guide-jp/flags.md)  
+- 🚩 [**フラグガイド**](docs/user-guide-jp/flags.md)  
   *動作制御パラメータ*
 
-- 🔧 [**MCPサーバー**](docs/User-Guide-jp/mcp-servers.md)  
+- 🔧 [**MCPサーバー**](docs/user-guide-jp/mcp-servers.md)  
   *6つのサーバー統合*
 
-- 💼 [**セッション管理**](docs/User-Guide-jp/session-management.md)  
+- 💼 [**セッション管理**](docs/user-guide-jp/session-management.md)  
   *状態の保存と復元*
 
 </td>
 <td valign="top">
 
-- 🏗️ [**技術アーキテクチャ**](docs/Developer-Guide/technical-architecture.md)  
+- 🏗️ [**技術アーキテクチャ**](docs/developer-guide/technical-architecture.md)  
   *システム設計の詳細*
 
-- 💻 [**コード貢献**](docs/Developer-Guide/contributing-code.md)  
+- 💻 [**コード貢献**](docs/developer-guide/contributing-code.md)  
   *開発ワークフロー*
 
-- 🧪 [**テスト＆デバッグ**](docs/Developer-Guide/testing-debugging.md)  
+- 🧪 [**テスト＆デバッグ**](docs/developer-guide/testing-debugging.md)  
   *品質保証*
 
 </td>
 <td valign="top">
 
-- ✨ [**ベストプラクティス**](docs/Reference/quick-start-practices.md)  
+- ✨ [**ベストプラクティス**](docs/reference/quick-start-practices.md)  
   *プロのコツとパターン*
 
-- 📓 [**サンプル集**](docs/Reference/examples-cookbook.md)  
+- 📓 [**サンプル集**](docs/reference/examples-cookbook.md)  
   *実際の使用例*
 
-- 🔍 [**トラブルシューティング**](docs/Reference/troubleshooting.md)  
+- 🔍 [**トラブルシューティング**](docs/reference/troubleshooting.md)  
   *一般的な問題と修正*
 
 </td>