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>

src/skill_seekers/mcp/tools/splitting_tools.py

@@ -94,17 +94,22 @@ def run_subprocess_with_streaming(cmd, timeout=None):
 
 async def split_config(args: dict) -> List[TextContent]:
     """
-    Split large documentation config into multiple focused skills.
+    Split large configs into multiple focused skills.
+
+    Supports both documentation and unified (multi-source) configs:
+    - Documentation configs: Split by categories, size, or create router skills
+    - Unified configs: Split by source type (documentation, github, pdf)
 
     For large documentation sites (10K+ pages), this tool splits the config into
-    multiple smaller configs based on categories, size, or custom strategy. This
-    improves performance and makes individual skills more focused.
+    multiple smaller configs. For unified configs with multiple sources, splits
+    into separate configs per source type.
 
     Args:
         args: Dictionary containing:
-            - config_path (str): Path to config JSON file (e.g., configs/godot.json)
-            - strategy (str, optional): Split strategy: auto, none, category, router, size (default: auto)
-            - target_pages (int, optional): Target pages per skill (default: 5000)
+            - config_path (str): Path to config JSON file (e.g., configs/godot.json or configs/react_unified.json)
+            - strategy (str, optional): Split strategy: auto, none, source, category, router, size (default: auto)
+                                       'source' strategy is for unified configs only
+            - target_pages (int, optional): Target pages per skill for doc configs (default: 5000)
             - dry_run (bool, optional): Preview without saving files (default: False)
 
     Returns: