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>

docs/Development/tasks/current-tasks.md

@@ -20,22 +20,22 @@
 
 ### Phase 1: ドキュメント基盤整備
 - [x] **PM Agent理想ワークフローをドキュメント化**
-  - File: `docs/Development/pm-agent-ideal-workflow.md`
+  - File: `docs/development/pm-agent-ideal-workflow.md`
   - Content: 完璧なワークフロー（7フェーズ）
   - Purpose: 次回セッションで同じ説明を繰り返さない
 
 - [x] **プロジェクト構造理解をドキュメント化**
-  - File: `docs/Development/project-structure-understanding.md`
+  - File: `docs/development/project-structure-understanding.md`
   - Content: Git管理とインストール後環境の区別
   - Purpose: 何百回も説明した内容を外部化
 
 - [x] **インストールフロー理解をドキュメント化**
-  - File: `docs/Development/installation-flow-understanding.md`
+  - File: `docs/development/installation-flow-understanding.md`
   - Content: CommandsComponent動作の完全理解
   - Source: `superclaude/commands/*.md` → `~/.claude/commands/sc/*.md`
 
 - [x] **ディレクトリ構造作成**
-  - `docs/Development/tasks/` - タスク管理
+  - `docs/development/tasks/` - タスク管理
   - `docs/patterns/` - 成功パターン記録
   - `docs/mistakes/` - 失敗記録と防止策
 
@@ -58,7 +58,7 @@
 - [ ] **改善提案ドキュメント作成**
   - Status: Pending
   - Action: 仮説ドキュメント作成
-  - File: `docs/Development/hypothesis-pm-enhancement-2025-10-14.md`
+  - File: `docs/development/hypothesis-pm-enhancement-2025-10-14.md`
   - Content:
     - 現状の問題点（ドキュメント寄り、PMO機能不足）
     - 改善案（自律的PDCA、自己評価）
@@ -130,7 +130,7 @@
   - テスト時のバックアップ・復元必須
 
 - **ドキュメント駆動開発**
-  - 理解 → docs/Development/ に記録
+  - 理解 → docs/development/ に記録
   - 仮説 → hypothesis-*.md
   - 実験 → experiment-*.md
   - 成功 → docs/patterns/