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/shell-integration/startup-scripts/init-posix.sh

@@ -47,6 +47,10 @@ function uv() {
   wrapSafeChainCommand "uv" "$@"
 }
 
+function uvx() {
+  wrapSafeChainCommand "uvx" "$@"
+}
+
 function poetry() {
   wrapSafeChainCommand "poetry" "$@"
 }