Support mithril-cli as a first-class entrypoint target

Create a `bin/run-mithril` script and wire it up to the entrypoint script. Make sure it configures the correct environment variables and supports both of the mutually exclusive workflows; simply setting NETWORK and the more explicit set of environment variables.

Start by locating the repository's entrypoint script and reviewing how existing entrypoint targets configure environment variables. Add the requested bin/run-mithril target and verify both mutually exclusive workflows: NETWORK alone, and the explicit environment-variable set; done means each selects the intended configuration without conflicting variables.

Written by the indexing model from the issue text.