feat(multi-llm): Phase 1 - Foundation adaptor architecture

Implement base adaptor pattern for multi-LLM support (Issue #179)

**Architecture:**
- Created adaptors/ package with base SkillAdaptor class
- Implemented factory pattern with get_adaptor() registry
- Refactored Claude-specific code into ClaudeAdaptor

**Changes:**
- New: src/skill_seekers/cli/adaptors/base.py (SkillAdaptor + SkillMetadata)
- New: src/skill_seekers/cli/adaptors/__init__.py (registry + factory)
- New: src/skill_seekers/cli/adaptors/claude.py (refactored upload + enhance logic)
- Modified: package_skill.py (added --target flag, uses adaptor.package())
- Modified: upload_skill.py (added --target flag, uses adaptor.upload())
- Modified: enhance_skill.py (added --target flag, uses adaptor.enhance())

**Tests:**
- New: tests/test_adaptors/test_base.py (10 tests passing)
- All existing tests still pass (backward compatible)

**Backward Compatibility:**
- Default --target=claude maintains existing behavior
- All CLI tools work exactly as before without --target flag
- No breaking changes

**Next:** Phase 2 - Implement Gemini, OpenAI, Markdown adaptors

src/skill_seekers/cli/adaptors/__init__.py

@@ -0,0 +1,124 @@
+#!/usr/bin/env python3
+"""
+Multi-LLM Adaptor Registry
+
+Provides factory function to get platform-specific adaptors for skill generation.
+Supports Claude AI, Google Gemini, OpenAI ChatGPT, and generic Markdown export.
+"""
+
+from typing import Dict, Type
+
+from .base import SkillAdaptor, SkillMetadata
+
+# Import adaptors (some may not be implemented yet)
+try:
+    from .claude import ClaudeAdaptor
+except ImportError:
+    ClaudeAdaptor = None
+
+try:
+    from .gemini import GeminiAdaptor
+except ImportError:
+    GeminiAdaptor = None
+
+try:
+    from .openai import OpenAIAdaptor
+except ImportError:
+    OpenAIAdaptor = None
+
+try:
+    from .markdown import MarkdownAdaptor
+except ImportError:
+    MarkdownAdaptor = None
+
+
+# Registry of available adaptors
+ADAPTORS: Dict[str, Type[SkillAdaptor]] = {}
+
+# Register adaptors that are implemented
+if ClaudeAdaptor:
+    ADAPTORS['claude'] = ClaudeAdaptor
+if GeminiAdaptor:
+    ADAPTORS['gemini'] = GeminiAdaptor
+if OpenAIAdaptor:
+    ADAPTORS['openai'] = OpenAIAdaptor
+if MarkdownAdaptor:
+    ADAPTORS['markdown'] = MarkdownAdaptor
+
+
+def get_adaptor(platform: str, config: dict = None) -> SkillAdaptor:
+    """
+    Factory function to get platform-specific adaptor instance.
+
+    Args:
+        platform: Platform identifier ('claude', 'gemini', 'openai', 'markdown')
+        config: Optional platform-specific configuration
+
+    Returns:
+        SkillAdaptor instance for the specified platform
+
+    Raises:
+        ValueError: If platform is not supported or not yet implemented
+
+    Examples:
+        >>> adaptor = get_adaptor('claude')
+        >>> adaptor = get_adaptor('gemini', {'api_version': 'v1beta'})
+    """
+    if platform not in ADAPTORS:
+        available = ', '.join(ADAPTORS.keys())
+        if not ADAPTORS:
+            raise ValueError(
+                f"No adaptors are currently implemented. "
+                f"Platform '{platform}' is not available."
+            )
+        raise ValueError(
+            f"Platform '{platform}' is not supported or not yet implemented. "
+            f"Available platforms: {available}"
+        )
+
+    adaptor_class = ADAPTORS[platform]
+    return adaptor_class(config)
+
+
+def list_platforms() -> list[str]:
+    """
+    List all supported platforms.
+
+    Returns:
+        List of platform identifiers
+
+    Examples:
+        >>> list_platforms()
+        ['claude', 'gemini', 'openai', 'markdown']
+    """
+    return list(ADAPTORS.keys())
+
+
+def is_platform_available(platform: str) -> bool:
+    """
+    Check if a platform adaptor is available.
+
+    Args:
+        platform: Platform identifier to check
+
+    Returns:
+        True if platform is available
+
+    Examples:
+        >>> is_platform_available('claude')
+        True
+        >>> is_platform_available('unknown')
+        False
+    """
+    return platform in ADAPTORS
+
+
+# Export public interface
+__all__ = [
+    'SkillAdaptor',
+    'SkillMetadata',
+    'get_adaptor',
+    'list_platforms',
+    'is_platform_available',
+    'ADAPTORS',
+]