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/__init__.py

@@ -63,6 +63,19 @@ from .exceptions import (
     TokenCountError,
 )
 
+# Assembly
+from .assembly import (
+    ContextPipeline,
+    PipelineMetrics,
+)
+
+# Compression
+from .compression import (
+    ContextCompressor,
+    TruncationResult,
+    TruncationStrategy,
+)
+
 # Prioritization
 from .prioritization import (
     ContextRanker,
@@ -97,10 +110,17 @@ from .types import (
 )
 
 __all__ = [
+    # Assembly
+    "ContextPipeline",
+    "PipelineMetrics",
     # Budget Management
     "BudgetAllocator",
     "TokenBudget",
     "TokenCalculator",
+    # Compression
+    "ContextCompressor",
+    "TruncationResult",
+    "TruncationStrategy",
     # Configuration
     "ContextSettings",
     "get_context_settings",