refactor: extract transport-agnostic core and shared tool registry

Introduce aegis_gitea_mcp.registry as the single name->handler source of
truth consumed by every transport adapter, moving TOOL_HANDLERS out of the
FastAPI server module. Add aegis_gitea_mcp.errors.ToolError so core handlers
no longer import fastapi.HTTPException; raw_tools now raises ToolError and the
HTTP adapter maps it back to HTTPException, preserving status codes and audit
behavior. Add a subprocess boundary test asserting the core imports without
pulling in fastapi/uvicorn/starlette.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/aegis_gitea_mcp/errors.py

@@ -0,0 +1,25 @@
+"""Transport-agnostic error types raised by the core.
+
+Core tool handlers and the authorization layer must not depend on the web stack
+(FastAPI). They raise :class:`ToolError` carrying an advisory HTTP status code;
+each transport adapter maps it to its own wire format (the HTTP adapter to
+``fastapi.HTTPException``, the stdio adapter to an MCP error). This keeps the
+core importable without FastAPI installed.
+"""
+
+from __future__ import annotations
+
+
+class ToolError(Exception):
+    """Error raised by a core tool handler or the authorization layer.
+
+    Args:
+        message: Human-readable, non-sensitive error detail.
+        status_code: Advisory HTTP status (e.g. 403 for denied). Adapters map
+            this to their transport; the stdio adapter only uses the message.
+    """
+
+    def __init__(self, message: str, *, status_code: int = 400) -> None:
+        super().__init__(message)
+        self.status_code = status_code
+        self.detail = message