Summary

• validate skill source directories before syncing so empty or broken sources are rejected
• make copy fallback write to a temporary directory before replacing the app skill directory
• avoid deleting an existing real app skill directory just to attempt symlink creation in auto mode
• add a regression test for preserving an existing destination when the source is empty

Root cause

The sync path deleted the destination skill directory before attempting symlink creation. If symlink creation failed and fallback copy used an empty or invalid source directory, the previously working app skill directory had already been removed.

Validation

• pnpm typecheck
• pnpm test:unit
• pnpm format:check
• pnpm build:renderer
• git diff --check
• cargo test replace_dest_with_copy_rejects_empty_source_without_touching_existing_dest

Fixes #2762