[6.x] Clean Asset Meta Command

[jackmcdade]

Closes statamic/ideas#89

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Note

Medium Risk
Deletes files from asset container disks, so incorrect orphan detection or path handling could remove valid metadata; tests reduce but don’t eliminate that risk.

Overview
Adds a new statamic:assets:meta-clean console command to find and delete orphaned asset .meta/*.yaml files (optionally scoped to a single container) and to remove now-empty .meta directories afterward.

Includes a --dry-run mode to list what would be deleted without changing disks, registers the command in ConsoleServiceProvider, and adds coverage for deletion, preservation of valid metadata, container scoping, and root/nested .meta paths.

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