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/installation-flow-understanding.md

@@ -286,7 +286,7 @@ Read ~/.claude/commands/sc/pm.md
 ### Phase 3: 改善案作成
 ```bash
 # このプロジェクト内で（Git管理）
-Write docs/Development/hypothesis-pm-enhancement-2025-10-14.md
+Write docs/development/hypothesis-pm-enhancement-2025-10-14.md
 
 内容:
 - 現状の問題（ドキュメント寄りすぎ、PMO機能不足）