feat: Complete multi-platform feature parity implementation

This commit implements full feature parity across all platforms (Claude, Gemini, OpenAI, Markdown) and all skill modes (Docs, GitHub, PDF, Unified, Local Repo).

## Core Changes

### Phase 1: MCP Package Tool Multi-Platform Support
- Added `target` parameter to `package_skill_tool()` in packaging_tools.py
- Updated MCP server definition to expose `target` parameter
- Platform-specific packaging: ZIP for Claude/OpenAI/Markdown, tar.gz for Gemini
- Platform-specific output messages and instructions

### Phase 2: MCP Upload Tool Multi-Platform Support
- Added `target` parameter to `upload_skill_tool()` in packaging_tools.py
- Added optional `api_key` parameter for API key override
- Updated MCP server definition with platform selection
- Platform-specific API key validation (ANTHROPIC_API_KEY, GOOGLE_API_KEY, OPENAI_API_KEY)
- Graceful handling of Markdown (upload not supported)

### Phase 3: Standalone MCP Enhancement Tool
- Created new `enhance_skill_tool()` function (140+ lines)
- Supports both 'local' mode (Claude Code Max) and 'api' mode (platform APIs)
- Added MCP server definition for `enhance_skill`
- Works with Claude, Gemini, and OpenAI
- Integrated into MCP tools exports

### Phase 4: Unified Config Splitting Support
- Added `is_unified_config()` method to detect multi-source configs
- Implemented `split_by_source()` method to split by source type (docs, github, pdf)
- Updated auto-detection to recommend 'source' strategy for unified configs
- Added 'source' to valid CLI strategy choices
- Updated MCP tool documentation for unified support

### Phase 5: Comprehensive Feature Matrix Documentation
- Created `docs/FEATURE_MATRIX.md` (~400 lines)
- Complete platform comparison tables
- Skill mode support matrix
- CLI and MCP tool coverage matrices
- Platform-specific notes and FAQs
- Workflow examples for each combination
- Updated README.md with feature matrix section

## Files Modified

**Core Implementation:**
- src/skill_seekers/mcp/tools/packaging_tools.py
- src/skill_seekers/mcp/server_fastmcp.py
- src/skill_seekers/mcp/tools/__init__.py
- src/skill_seekers/cli/split_config.py
- src/skill_seekers/mcp/tools/splitting_tools.py

**Documentation:**
- docs/FEATURE_MATRIX.md (NEW)
- README.md

**Tests:**
- tests/test_install_multiplatform.py (already existed)

## Test Results
- ✅ 699 tests passing
- ✅ All multiplatform install tests passing (6/6)
- ✅ No regressions introduced
- ✅ All syntax checks passed
- ✅ Import tests successful

## Breaking Changes
None - all changes are backward compatible with default `target='claude'`

## Migration Guide
Existing MCP calls without `target` parameter will continue to work (defaults to 'claude').

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

README.md

@@ -303,6 +303,39 @@ skill-seekers install --config react
 
 ---
 
+## 📊 Feature Matrix
+
+Skill Seekers supports **4 platforms** and **5 skill modes** with full feature parity.
+
+**Platforms:** Claude AI, Google Gemini, OpenAI ChatGPT, Generic Markdown
+**Skill Modes:** Documentation, GitHub, PDF, Unified Multi-Source, Local Repository
+
+See [Complete Feature Matrix](docs/FEATURE_MATRIX.md) for detailed platform and feature support.
+
+### Quick Platform Comparison
+
+| Feature | Claude | Gemini | OpenAI | Markdown |
+|---------|--------|--------|--------|----------|
+| Format | ZIP + YAML | tar.gz | ZIP + Vector | ZIP |
+| Upload | ✅ API | ✅ API | ✅ API | ❌ Manual |
+| Enhancement | ✅ Sonnet 4 | ✅ 2.0 Flash | ✅ GPT-4o | ❌ None |
+| All Skill Modes | ✅ | ✅ | ✅ | ✅ |
+
+**Examples:**
+```bash
+# Package for all platforms (same skill)
+skill-seekers package output/react/ --target claude
+skill-seekers package output/react/ --target gemini
+skill-seekers package output/react/ --target openai
+skill-seekers package output/react/ --target markdown
+
+# Install for specific platform
+skill-seekers install --config django --target gemini
+skill-seekers install --config fastapi --target openai
+```
+
+---
+
 ## Usage Examples
 
 ### Documentation Scraping