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

tests/test_adaptors/__init__.py

@@ -0,0 +1 @@
+# Adaptor tests package

tests/test_adaptors/test_base.py

@@ -0,0 +1,122 @@
+#!/usr/bin/env python3
+"""
+Tests for base adaptor and registry
+"""
+
+import unittest
+from pathlib import Path
+
+from skill_seekers.cli.adaptors import (
+    get_adaptor,
+    list_platforms,
+    is_platform_available,
+    SkillAdaptor,
+    SkillMetadata,
+    ADAPTORS
+)
+
+
+class TestSkillMetadata(unittest.TestCase):
+    """Test SkillMetadata dataclass"""
+
+    def test_basic_metadata(self):
+        """Test basic metadata creation"""
+        metadata = SkillMetadata(
+            name="test-skill",
+            description="Test skill description"
+        )
+
+        self.assertEqual(metadata.name, "test-skill")
+        self.assertEqual(metadata.description, "Test skill description")
+        self.assertEqual(metadata.version, "1.0.0")  # default
+        self.assertIsNone(metadata.author)  # default
+        self.assertEqual(metadata.tags, [])  # default
+
+    def test_full_metadata(self):
+        """Test metadata with all fields"""
+        metadata = SkillMetadata(
+            name="react",
+            description="React documentation",
+            version="2.0.0",
+            author="Test Author",
+            tags=["react", "javascript", "web"]
+        )
+
+        self.assertEqual(metadata.name, "react")
+        self.assertEqual(metadata.description, "React documentation")
+        self.assertEqual(metadata.version, "2.0.0")
+        self.assertEqual(metadata.author, "Test Author")
+        self.assertEqual(metadata.tags, ["react", "javascript", "web"])
+
+
+class TestAdaptorRegistry(unittest.TestCase):
+    """Test adaptor registry and factory"""
+
+    def test_list_platforms(self):
+        """Test listing available platforms"""
+        platforms = list_platforms()
+
+        self.assertIsInstance(platforms, list)
+        # Claude should always be available
+        self.assertIn('claude', platforms)
+
+    def test_is_platform_available(self):
+        """Test checking platform availability"""
+        # Claude should be available
+        self.assertTrue(is_platform_available('claude'))
+
+        # Unknown platform should not be available
+        self.assertFalse(is_platform_available('unknown_platform'))
+
+    def test_get_adaptor_claude(self):
+        """Test getting Claude adaptor"""
+        adaptor = get_adaptor('claude')
+
+        self.assertIsInstance(adaptor, SkillAdaptor)
+        self.assertEqual(adaptor.PLATFORM, 'claude')
+        self.assertEqual(adaptor.PLATFORM_NAME, 'Claude AI (Anthropic)')
+
+    def test_get_adaptor_invalid(self):
+        """Test getting invalid adaptor raises error"""
+        with self.assertRaises(ValueError) as ctx:
+            get_adaptor('invalid_platform')
+
+        error_msg = str(ctx.exception)
+        self.assertIn('invalid_platform', error_msg)
+        self.assertIn('not supported', error_msg)
+
+    def test_get_adaptor_with_config(self):
+        """Test getting adaptor with custom config"""
+        config = {'custom_setting': 'value'}
+        adaptor = get_adaptor('claude', config)
+
+        self.assertEqual(adaptor.config, config)
+
+
+class TestBaseAdaptorInterface(unittest.TestCase):
+    """Test base adaptor interface methods"""
+
+    def setUp(self):
+        """Set up test adaptor"""
+        self.adaptor = get_adaptor('claude')
+
+    def test_validate_api_key_default(self):
+        """Test default API key validation"""
+        # Claude adaptor overrides this
+        self.assertTrue(self.adaptor.validate_api_key('sk-ant-test123'))
+        self.assertFalse(self.adaptor.validate_api_key('invalid'))
+
+    def test_get_env_var_name(self):
+        """Test environment variable name"""
+        env_var = self.adaptor.get_env_var_name()
+
+        self.assertEqual(env_var, 'ANTHROPIC_API_KEY')
+
+    def test_supports_enhancement(self):
+        """Test enhancement support check"""
+        # Claude supports enhancement
+        self.assertTrue(self.adaptor.supports_enhancement())
+
+
+if __name__ == '__main__':
+    unittest.main()