feat(mcp): per-request user_id contextvar for tool handlers

Adds mcp._context.current_user_id() backed by a ContextVar. The ASGI
auth middleware sets it before dispatching to FastMCP and resets it
on the way out, so tool handlers can read the acting user without
re-parsing the request scope.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/fabledassistant/mcp/_context.py

@@ -0,0 +1,20 @@
+"""Per-request MCP context.
+
+The ASGI middleware (mcp/server.py) populates `_user_id_ctx` from
+scope['fable_user_id'] before dispatching to FastMCP tool handlers.
+Tool functions then call `current_user_id()` to retrieve it.
+
+Tools must never fall back to a default user — `current_user_id()` raises
+if called outside a properly-authed MCP request.
+"""
+from contextvars import ContextVar
+
+_user_id_ctx: ContextVar[int | None] = ContextVar("fable_mcp_user_id", default=None)
+
+
+def current_user_id() -> int:
+    """Return the user_id for the in-flight MCP request. Raises if unset."""
+    uid = _user_id_ctx.get()
+    if uid is None:
+        raise RuntimeError("no MCP user context — request was not bearer-authed")
+    return uid