Function resolveFirstPartyPlugin

Import a first-party package's advertised CLI plugin.

Reads the package's package.json stone.cliPlugin contract, imports the referenced module (from the project's own node_modules, i.e. a direct dependency, never a transitive one) and validates the result. Returns undefined when the package advertises no contract, the module cannot be imported, or the export is not a plugin, so a broken dependency never breaks the build.