Refactor project structure and enhance backend integration

- Expanded package inclusion in `pyproject.toml` to support new modules.
- Introduced new `adapters` and `protocol` packages for better organization.
- Added backend adapter implementations for control plane integration.
- Updated main application imports to reflect new package structure.
- Removed deprecated core components and adjusted documentation accordingly.
- Enhanced architecture documentation to clarify the new runtime and integration layers.

engine/runtime/session/metadata.py

@@ -0,0 +1,9 @@
+"""Metadata helpers extracted from session manager."""
+
+from __future__ import annotations
+
+import re
+from typing import Pattern
+
+DYNAMIC_VARIABLE_KEY_RE: Pattern[str] = re.compile(r"^[a-zA-Z_][a-zA-Z0-9_]{0,63}$")
+DYNAMIC_VARIABLE_PLACEHOLDER_RE: Pattern[str] = re.compile(r"\{\{\s*([a-zA-Z_][a-zA-Z0-9_]*)\s*\}\}")