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>
