Unified target command: run, with exec for ad-hoc commands

The CLI verbs have been simplified. The old fabrik build is now fabrik run //pkg:name, which produces the named target’s outputs uniformly across target kinds. The previous ad-hoc fabrik run -- <command> escape hatch has been renamed to fabrik exec -- <command> and is now the substrate-level way to cache and run arbitrary commands without touching the target graph.