Add uvx support

Add uvx as a supported package manager so that `uvx` commands are
routed through safe-chain's MITM proxy for malware detection, just
like `uv`. Previously, `uvx` bypassed all safe-chain protections.

The uvx package manager reuses the existing uv command runner since
uvx is functionally equivalent to `uv tool run`.

Fixes #268

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/safe-chain/src/packagemanager/uvx/createUvxPackageManager.js

@@ -0,0 +1,18 @@
+import { runUv } from "../uv/runUvCommand.js";
+
+/**
+ * @returns {import("../currentPackageManager.js").PackageManager}
+ */
+export function createUvxPackageManager() {
+  return {
+    /**
+     * @param {string[]} args
+     */
+    runCommand: (args) => {
+      return runUv("uvx", args);
+    },
+    // For uvx, rely solely on MITM
+    isSupportedCommand: () => false,
+    getDependencyUpdatesForCommand: () => [],
+  };
+}