feat: add project adapter shadow contract

src/docforge/models.py

@@ -4,6 +4,7 @@ from __future__ import annotations
 
 from dataclasses import asdict, dataclass
 from pathlib import Path
+from typing import Protocol
 
 
 @dataclass(frozen=True)
@@ -118,6 +119,14 @@ class ProjectSnapshot:
     revision: str
 
 
+class ProjectService(Protocol):
+    """Minimum immutable project boundary required by derived read services."""
+
+    descriptor: ProjectDescriptor
+
+    def load(self) -> ProjectSnapshot: ...
+
+
 @dataclass(frozen=True)
 class ContextEntry:
     node_id: str