--exclude=sys matched every path component named sys anywhere in the
tree — GNU tar applies slash-less patterns to all components — and so
silently dropped usr/share/go-1.24/src/internal/runtime/sys from the
archive, breaking every go build in the sandbox with 'package
internal/runtime/sys is not in std'. The excludes are now anchored
(./proc, ./sys, ./dev), so only the top-level mountpoints stay out.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>