feat(context): Phase 1 - Foundation types, config and exceptions (#79)

Implements the foundation for Context Management Engine:

Types (backend/app/services/context/types/):
- BaseContext: Abstract base with ID, content, priority, scoring
- SystemContext: System prompts, personas, instructions
- KnowledgeContext: RAG results from Knowledge Base MCP
- ConversationContext: Chat history with role support
- TaskContext: Task/issue context with acceptance criteria
- ToolContext: Tool definitions and execution results
- AssembledContext: Final assembled context result

Configuration (config.py):
- Token budget allocation (system 5%, task 10%, knowledge 40%, etc.)
- Scoring weights (relevance 50%, recency 30%, priority 20%)
- Cache settings (TTL, prefix)
- Performance settings (max assembly time, parallel scoring)
- Environment variable overrides with CTX_ prefix

Exceptions (exceptions.py):
- ContextError: Base exception
- BudgetExceededError: Token budget violations
- TokenCountError: Token counting failures
- CompressionError: Compression failures
- AssemblyTimeoutError: Assembly timeout
- ScoringError, FormattingError, CacheError
- ContextNotFoundError, InvalidContextError

All 86 tests pass.

🤖 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

@@ -0,0 +1,5 @@
+"""
+Context Compression Module.
+
+Provides truncation and compression strategies.
+"""