feat: Add OurDigital custom skills package (10 skills)

Complete implementation of OurDigital skills with dual-platform support
(Claude Desktop + Claude Code) following standardized structure.

Skills created:
- 01-ourdigital-brand-guide: Brand reference & style guidelines
- 02-ourdigital-blog: Korean blog drafts (blog.ourdigital.org)
- 03-ourdigital-journal: English essays (journal.ourdigital.org)
- 04-ourdigital-research: Research prompts & workflows
- 05-ourdigital-document: Notion-to-presentation pipeline
- 06-ourdigital-designer: Visual/image prompt generation
- 07-ourdigital-ad-manager: Ad copywriting & keyword research
- 08-ourdigital-trainer: Training materials & workshop planning
- 09-ourdigital-backoffice: Quotes, proposals, cost analysis
- 10-ourdigital-skill-creator: Meta skill for creating new skills

Features:
- YAML frontmatter with "ourdigital" or "our" prefix triggers
- Standardized directory structure (code/, desktop/, shared/, docs/)
- Shared environment setup (_ourdigital-shared/)
- Comprehensive reference documentation
- Cross-skill integration support

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

custom-skills/_ourdigital-shared/requirements/base.txt

@@ -0,0 +1,29 @@
+# OurDigital Skills - Base Requirements
+# Common dependencies for all ourdigital skills
+
+# Core utilities
+requests>=2.31.0
+python-dotenv>=1.0.0
+pyyaml>=6.0.1
+
+# Notion integration
+notion-client>=2.2.0
+
+# Date/time handling
+python-dateutil>=2.8.2
+
+# File handling
+pathlib2>=2.3.7;python_version<"3.12"
+
+# JSON handling
+orjson>=3.9.0
+
+# Markdown processing
+markdown>=3.5.0
+markdownify>=0.11.6
+
+# HTTP client
+httpx>=0.25.0
+
+# Data validation
+pydantic>=2.5.0