feat(A1.9): Add multi-source git repository support for config fetching

This major feature enables fetching configs from private/team git repositories
in addition to the public API, unlocking team collaboration and custom config
collections.

**New Components:**
- git_repo.py (283 lines): GitConfigRepo class for git operations
  - Shallow clone/pull with GitPython
  - Config discovery (recursive *.json search)
  - Token injection for private repos
  - Comprehensive error handling

- source_manager.py (260 lines): SourceManager class for registry
  - Add/list/remove config sources
  - Priority-based resolution
  - Atomic file I/O
  - Auto-detect token env vars

**MCP Integration:**
- Enhanced fetch_config: 3 modes (API, Git URL, Named Source)
- New tools: add_config_source, list_config_sources, remove_config_source
- Backward compatible: existing API mode unchanged

**Testing:**
- 83 tests (100% passing)
  - 35 tests for GitConfigRepo
  - 48 tests for SourceManager
  - Integration tests for MCP tools
- Comprehensive error scenarios covered

**Dependencies:**
- Added GitPython>=3.1.40

**Architecture:**
- Storage: ~/.skill-seekers/sources.json (registry)
- Cache: $SKILL_SEEKERS_CACHE_DIR (default: ~/.skill-seekers/cache/)
- Auth: Environment variables only (GITHUB_TOKEN, GITLAB_TOKEN, etc.)

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

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

pyproject.toml

@@ -42,6 +42,7 @@ dependencies = [
     "requests>=2.32.5",
     "beautifulsoup4>=4.14.2",
     "PyGithub>=2.5.0",
+    "GitPython>=3.1.40",
     "mcp>=1.18.0",
     "httpx>=0.28.1",
     "httpx-sse>=0.4.3",