Add remote-site cache branching

[adamziel]

Summary

• add a production forkpress remote command for registering, listing, showing, and branching from remote-site caches
• add storage support for remote-site cache manifests and wp-cow file-cache/mirror reuse without deleting or moving accumulated cache entries
• add a COW helper to create normal ForkPress branches from an existing materialized WordPress tree

Validation

• cargo fmt --check
• cargo check -p forkpress-storage
• FORKPRESS_RUNTIME_BUNDLE=/dev/null cargo check -p forkpress-cli --bin forkpress
• cargo test -p forkpress-storage
• FORKPRESS_RUNTIME_BUNDLE=/dev/null cargo test -p forkpress-cli --bin forkpress
• cargo test --workspace --exclude forkpress-cli
• FORKPRESS_RUNTIME_BUNDLE=/dev/null cargo test -p forkpress-cli parses_remote_add_wp_cow_clone --bin forkpress