feat(context): implement model adapters for Claude and OpenAI (#83)

Phase 5 of Context Management Engine - Model Adapters:

- Add ModelAdapter abstract base class with model matching
- Add DefaultAdapter for unknown models (plain text)
- Add ClaudeAdapter with XML-based formatting:
  - <system_instructions> for system context
  - <reference_documents>/<document> for knowledge
  - <conversation_history>/<message> for chat
  - <tool_results>/<tool_result> for tool outputs
  - XML escaping for special characters
- Add OpenAIAdapter with markdown formatting:
  - ## headers for sections
  - ### Source headers for documents
  - **ROLE** bold labels for conversation
  - Code blocks for tool outputs
- Add get_adapter() factory function for model selection

Tests: 33 new tests, 256 total context tests passing

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

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

backend/app/services/context/adapters/__init__.py

@@ -1,5 +1,35 @@
 """
 Model Adapters Module.
 
-Provides model-specific context formatting.
+Provides model-specific context formatting adapters.
 """
+
+from .base import DefaultAdapter, ModelAdapter
+from .claude import ClaudeAdapter
+from .openai import OpenAIAdapter
+
+
+def get_adapter(model: str) -> ModelAdapter:
+    """
+    Get the appropriate adapter for a model.
+
+    Args:
+        model: Model name
+
+    Returns:
+        Adapter instance for the model
+    """
+    if ClaudeAdapter.matches_model(model):
+        return ClaudeAdapter()
+    elif OpenAIAdapter.matches_model(model):
+        return OpenAIAdapter()
+    return DefaultAdapter()
+
+
+__all__ = [
+    "ClaudeAdapter",
+    "DefaultAdapter",
+    "get_adapter",
+    "ModelAdapter",
+    "OpenAIAdapter",
+]