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/__init__.py

@@ -0,0 +1,105 @@
+"""
+Context Management Engine
+
+Sophisticated context assembly and optimization for LLM requests.
+Provides intelligent context selection, token budget management,
+and model-specific formatting.
+
+Usage:
+    from app.services.context import (
+        ContextSettings,
+        get_context_settings,
+        SystemContext,
+        KnowledgeContext,
+        ConversationContext,
+        TaskContext,
+        ToolContext,
+    )
+
+    # Get settings
+    settings = get_context_settings()
+
+    # Create context instances
+    system_ctx = SystemContext.create_persona(
+        name="Code Assistant",
+        description="You are a helpful code assistant.",
+        capabilities=["Write code", "Debug issues"],
+    )
+"""
+
+# Configuration
+from .config import (
+    ContextSettings,
+    get_context_settings,
+    get_default_settings,
+    reset_context_settings,
+)
+
+# Exceptions
+from .exceptions import (
+    AssemblyTimeoutError,
+    BudgetExceededError,
+    CacheError,
+    CompressionError,
+    ContextError,
+    ContextNotFoundError,
+    FormattingError,
+    InvalidContextError,
+    ScoringError,
+    TokenCountError,
+)
+
+# Types
+from .types import (
+    AssembledContext,
+    BaseContext,
+    ContextPriority,
+    ContextType,
+    ConversationContext,
+    KnowledgeContext,
+    MessageRole,
+    SystemContext,
+    TaskComplexity,
+    TaskContext,
+    TaskStatus,
+    ToolContext,
+    ToolResultStatus,
+)
+
+__all__ = [
+    # Configuration
+    "ContextSettings",
+    "get_context_settings",
+    "get_default_settings",
+    "reset_context_settings",
+    # Exceptions
+    "AssemblyTimeoutError",
+    "BudgetExceededError",
+    "CacheError",
+    "CompressionError",
+    "ContextError",
+    "ContextNotFoundError",
+    "FormattingError",
+    "InvalidContextError",
+    "ScoringError",
+    "TokenCountError",
+    # Types - Base
+    "AssembledContext",
+    "BaseContext",
+    "ContextPriority",
+    "ContextType",
+    # Types - Conversation
+    "ConversationContext",
+    "MessageRole",
+    # Types - Knowledge
+    "KnowledgeContext",
+    # Types - System
+    "SystemContext",
+    # Types - Task
+    "TaskComplexity",
+    "TaskContext",
+    "TaskStatus",
+    # Types - Tool
+    "ToolContext",
+    "ToolResultStatus",
+]