feat(cli): add `mpak provider unset` command

[mgoldsborough]

Problem

mpak provider set <name> persists a default provider to ~/.mpak/config.json, but there's no way to clear it back to auto-detect without manually editing the config file.

Proposal

Add mpak provider unset that removes the provider field from config, restoring auto-detect behavior.

mpak provider unset
# → Default provider cleared. Auto-detection will be used.

Context

From QA review of #feat/multi-provider-skill-install. Non-blocking for that PR but improves the UX of the provider system.