feat: add adapter read-only MCP boundary

src/docforge/adapter_contract.py

@@ -227,6 +227,11 @@ class AdapterProject:
             revision=projection.revision,
         )
 
+    def canonical_source_paths(self) -> tuple[Path, ...]:
+        """Adapters validate their own source sets before producing a projection."""
+
+        return ()
+
 
 @dataclass(frozen=True)
 class ShadowArtifact: