[NFR]: crest new runs the commands that crest.php lists

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Summary

crest.php can list commands that crest new runs after it writes the project.

Why

crest new runs nothing today: no composer, no docker, no network. This is a decision: a run that writes the files and then fails in composer is worse than printing the next commands. A list in crest.php keeps that default, and lets a user choose more.

Scope

• A key in crest.php (required after [NFR]: crest init and a required crest.php #28) lists commands, in order. For example composer install, or crest up and crest install.
• An empty list, the default, keeps today's behavior: new runs nothing.
• new writes all the files first, then runs the commands. The first command that fails stops the list, with a message that names it. The files stay.

Open

• The key name and its form.
• Where the list comes from before the project exists: a published project-config stub (crest stub:publish), or the questions of new ([NFR]: crest init and a required crest.php #28).
• How this fits with the one-line setup for docker-only users.

Done when

A list in crest.php makes new run those commands in order, and stop at the first failure with a clear message.

Related: #28, #8.