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_STATUS.md

@@ -48,11 +48,11 @@
 - ✅ Agents/pm-agent.md updated
 
 ### Documentation
-- ✅ docs/Development/ARCHITECTURE.md
-- ✅ docs/Development/ROADMAP.md
-- ✅ docs/Development/TASKS.md
-- ✅ docs/Development/PROJECT_STATUS.md
-- ✅ docs/pm-agent-implementation-status.md
+- ✅ docs/development/ARCHITECTURE.md
+- ✅ docs/development/ROADMAP.md
+- ✅ docs/development/TASKS.md
+- ✅ docs/development/PROJECT_STATUS.md
+- ✅ docs/PM_AGENT.md
 
 ---
 
@@ -135,7 +135,7 @@
 ### Week 2-3
 - [ ] Implement PM Agent Core (session_lifecycle, pdca_engine, memory_ops)
 - [ ] Write unit tests
-- [ ] Update User-Guide documentation
+- [ ] Update user-guide documentation
 
 ### Week 4-5
 - [ ] Configure Serena MCP server
@@ -147,7 +147,7 @@
 ## 📝 Recent Changes
 
 ### 2025-10-14
-- Created docs/Development/ structure
+- Created docs/development/ structure
 - Wrote ARCHITECTURE.md (system overview)
 - Wrote ROADMAP.md (5-phase development plan)
 - Wrote TASKS.md (task tracking)