build: keep mise.macos-x64.lock in sync when mr-boxington is bumped #138

PR: #137
Review thread: #137 (comment)

Problem

mise run lock (added in #137) refreshes mise.lock but never mise.macos-x64.lock. On mise 2026.11.1 no command can rewrite that file once mise.lock holds the release-asset entry for mr-boxington. With .miserc.toml's auto_env on, an Intel Mac also targets macos-x64 for the base lockfile. That fails with failed to resolve mr-boxington for macos-x64: selecting the packslip artifact for macos-x64, because the release has no darwin x86_64 asset. MISE_ENV=macos-x64 loads mise.macos-x64.toml but writes to mise.lock.

The file records only the version (cargo:mbx builds from source, so there is no checksum). It goes stale only when the mr-boxington version changes, and then it has to be edited by hand.

Proposal

• Recheck on a newer mise whether a lock of the platform config can skip the base lockfile's macos-x64 target, or honour the cargo:mbx alias there.
• If it can, add that step to [tasks.lock].
• If it still cannot, document the manual edit next to the mr-boxington pin, or drop mise.macos-x64.lock. It carries no checksum, so it adds no verification.