Remote-backed cache via pluggable provider with Tuist

Fabrik now ships a pluggable cache provider layer, with Tuist as the first remote backend. Workspaces declare the provider in fabrik.toml as local, a named entry, or a direct tuist block (url, endpoint, account, project, token_env, oauth_client_id), defaulting to https://tuist.dev and TUIST_TOKEN. Writes mirror to the remote while a local CAS keeps serving hot reads, so build, run, test, exec, cache, and deps output can be shared across machines and CI runs. A new fabrik auth login|logout <provider> command drives OAuth login through the browser and stores credentials under XDG.