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/project-structure-understanding.md

@@ -146,7 +146,7 @@ cat ~/.claude/commands/sc/pm.md      # 現在の仕様確認
 cd ~/github/SuperClaude_Framework
 
 # Git管理されているこのプロジェクト内で
-Write docs/Development/hypothesis-pm-improvement-YYYY-MM-DD.md
+Write docs/development/hypothesis-pm-improvement-YYYY-MM-DD.md
 
 # 内容例:
 # - 現状の問題
@@ -226,9 +226,9 @@ mv backup original
 ```
 
 ### Rule 3: ドキュメント駆動開発
-1. 理解 → docs/Development/ に記録
-2. 仮説 → docs/Development/hypothesis-*.md
-3. 実験 → docs/Development/experiment-*.md
+1. 理解 → docs/development/ に記録
+2. 仮説 → docs/development/hypothesis-*.md
+3. 実験 → docs/development/experiment-*.md
 4. 成功 → docs/patterns/
 5. 失敗 → docs/mistakes/
 
@@ -336,7 +336,7 @@ vim ~/.claude/commands/sc/pm.md
 ```bash
 # ✅ CORRECT
 1. setup/components/ でロジック理解
-2. docs/Development/ に改善案記録
+2. docs/development/ に改善案記録
 3. setup/ 側で変更・テスト
 4. Git コミット
 5. SuperClaude install --dev で動作確認
@@ -357,7 +357,7 @@ vim ~/.claude/commands/sc/pm.md
    - 現在の動作理解
 
 3. **改善提案作成**
-   - `docs/Development/hypothesis-*.md` 作成
+   - `docs/development/hypothesis-*.md` 作成
    - ユーザーレビュー
 
 4. **実装・テスト**