Fix orphaned fileHashes blocking pack removal

[bguidolim]

Context

mcs sync --global repeatedly tries to remove packs that are already gone, printing:

[WARN] Some artifacts for <pack> could not be removed. Re-run 'mcs sync --global' to retry.

Root cause: unconfigurePack() removes entries from remaining.files after successful file deletion but never cleans up remaining.fileHashes. Since PackArtifactRecord.isEmpty checks fileHashes.isEmpty, orphaned hash entries (present in fileHashes but absent from files) permanently prevent the pack from being fully removed from state.

This data inconsistency was introduced by the conditional copyPackFile namespacing feature (aa7fd9a), which could remove files from files during reconciliation while leaving stale fileHashes behind.

Changes

• After clearing remaining.files in unconfigurePack(), filter remaining.fileHashes to only keep entries whose key exists in remaining.files — handles both normal removal and orphaned state
• Added test that seeds a state file with the exact broken pattern (empty files, non-empty fileHashes) and verifies the pack is fully removed

Testing

• All 876 tests pass
• New test unconfigureClearsOrphanedFileHashes covers the exact scenario
• SwiftFormat + SwiftLint clean