Variable precedence and overrides

Precedence (lowest → highest): vars declaration → gatherer result → --var-file → --var.

- --var KEY=VALUE parses VALUE as a WCL expression when possible (`--var count=3` is an int), falling back to a plain string. Repeatable. - --var-file file.wcl is a flat name = value collection; expressions evaluate standalone and cannot reference other variables. - Gather params evaluate before variables resolve: they may reference --var / --var-file overrides, but not gatherer results or vars that depend on them. - Gatherer invocations all run concurrently and are deduplicated by `(gatherer, canonicalized params)`; any gatherer failure aborts before step execution. - Conditions and properties evaluate lazily at run time against the full scope.