feat(multi-llm): Phase 2 - Gemini adaptor implementation
Implement Google Gemini platform support (Issue #179, Phase 2/6)
**Features:**
- Plain markdown format (no YAML frontmatter)
- tar.gz packaging for Gemini Files API
- Upload to Google AI Studio
- Enhancement using Gemini 2.0 Flash
- API key validation (AIza prefix)
**Implementation:**
- New: src/skill_seekers/cli/adaptors/gemini.py (430 lines)
- format_skill_md(): Plain markdown (no frontmatter)
- package(): Creates .tar.gz with system_instructions.md
- upload(): Uploads to Gemini Files API
- enhance(): Uses Gemini 2.0 Flash for enhancement
- validate_api_key(): Checks Google key format (AIza)
**Tests:**
- New: tests/test_adaptors/test_gemini_adaptor.py (13 tests)
- 11 passing unit tests
- 2 skipped (integration tests requiring real API keys)
- Tests: validation, formatting, packaging, error handling
**Test Summary:**
- Total adaptor tests: 23 (21 passing, 2 skipped)
- Base adaptor: 10 tests
- Gemini adaptor: 11 tests (2 skipped)
**Next:** Phase 3 - Implement OpenAI adaptor
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