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/__init__.py

@@ -76,6 +76,15 @@ from .compression import (
     TruncationStrategy,
 )
 
+# Adapters
+from .adapters import (
+    ClaudeAdapter,
+    DefaultAdapter,
+    get_adapter,
+    ModelAdapter,
+    OpenAIAdapter,
+)
+
 # Prioritization
 from .prioritization import (
     ContextRanker,
@@ -110,6 +119,12 @@ from .types import (
 )
 
 __all__ = [
+    # Adapters
+    "ClaudeAdapter",
+    "DefaultAdapter",
+    "get_adapter",
+    "ModelAdapter",
+    "OpenAIAdapter",
     # Assembly
     "ContextPipeline",
     "PipelineMetrics",