feat: establish read-only DocForge MCP foundation

src/docforge/errors.py

@@ -0,0 +1,16 @@
+"""Structured DocForge failures."""
+
+from __future__ import annotations
+
+
+class DocForgeError(Exception):
+    """A user-actionable project, source, validation, or cache failure."""
+
+    def __init__(self, code: str, message: str, **details: object) -> None:
+        super().__init__(message)
+        self.code = code
+        self.message = message
+        self.details = details
+
+    def as_dict(self) -> dict[str, object]:
+        return {"code": self.code, "message": self.message, "details": self.details}