Pluggable cache provider with Tuist backend

Workspaces can now choose a cache provider in fabrik.toml: keep builds entirely local, reference a named provider, or point directly at Tuist. Tuist is the first supported remote backend, mirroring cache writes while keeping a local content-addressed cache in front. You can authenticate with fabrik auth login <provider> and fabrik auth logout <provider>, which uses OAuth and stores credentials under the standard XDG directory. The resolved provider is now used across all existing commands: build, run, test, exec, cache, and deps.