chore(context): apply linter fixes and sort imports (#86)

Phase 8 of Context Management Engine - Final Cleanup:

- Sort __all__ exports alphabetically
- Sort imports per isort conventions
- Fix minor linting issues

Final test results:
- 311 context management tests passing
- 2507 total backend tests passing
- 85% code coverage

Context Management Engine is complete with all 8 phases:
1. Foundation: Types, Config, Exceptions
2. Token Budget Management
3. Context Scoring & Ranking
4. Context Assembly Pipeline
5. Model Adapters (Claude, OpenAI)
6. Caching Layer (Redis + in-memory)
7. Main Engine & Integration
8. Testing & Documentation

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

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

backend/app/services/context/engine.py

@@ -8,7 +8,6 @@ Provides a high-level API for assembling optimized context for LLM requests.
 import logging
 from typing import TYPE_CHECKING, Any
 
-from .adapters import get_adapter
 from .assembly import ContextPipeline
 from .budget import BudgetAllocator, TokenBudget, TokenCalculator
 from .cache import ContextCache