feat(multi-llm): Phase 3 - OpenAI adaptor implementation
Implement OpenAI ChatGPT platform support (Issue #179, Phase 3/6)
**Features:**
- Assistant instructions format (plain text, no frontmatter)
- ZIP packaging for Assistants API
- Upload creates Assistant + Vector Store with file_search
- Enhancement using GPT-4o
- API key validation (sk- prefix)
**Implementation:**
- New: src/skill_seekers/cli/adaptors/openai.py (520 lines)
- format_skill_md(): Assistant instructions format
- package(): Creates .zip with assistant_instructions.txt + vector_store_files/
- upload(): Creates Assistant with Vector Store via Assistants API
- enhance(): Uses GPT-4o for enhancement
- validate_api_key(): Checks OpenAI key format (sk-)
**Tests:**
- New: tests/test_adaptors/test_openai_adaptor.py (14 tests)
- 12 passing unit tests
- 2 skipped (integration tests requiring real API keys)
- Tests: validation, formatting, packaging, vector store structure
**Test Summary:**
- Total adaptor tests: 37 (33 passing, 4 skipped)
- Base: 10 tests
- Claude: (integrated in base)
- Gemini: 11 tests (2 skipped)
- OpenAI: 12 tests (2 skipped)
**Next:** Phase 4 - Implement Markdown adaptor (generic export)
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