feat(context): implement assembly pipeline and compression (#82)

Phase 4 of Context Management Engine - Assembly Pipeline:

- Add TruncationStrategy with end/middle/sentence-aware truncation
- Add TruncationResult dataclass for tracking compression metrics
- Add ContextCompressor for type-specific compression
- Add ContextPipeline orchestrating full assembly workflow:
  - Token counting for all contexts
  - Scoring and ranking via ContextRanker
  - Optional compression when budget threshold exceeded
  - Model-specific formatting (XML for Claude, markdown for OpenAI)
- Add PipelineMetrics for performance tracking
- Update AssembledContext with new fields (model, contexts, metadata)
- Add backward compatibility aliases for renamed fields

Tests: 34 new tests, 223 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/compression/__init__.py

@@ -3,3 +3,11 @@ Context Compression Module.
 
 Provides truncation and compression strategies.
 """
+
+from .truncation import ContextCompressor, TruncationResult, TruncationStrategy
+
+__all__ = [
+    "ContextCompressor",
+    "TruncationResult",
+    "TruncationStrategy",
+]