[Feature request] Enhance options for using herd init

[robw-raviga]

I run a team and we all use Herd, it's a super platform.

However, with the advent of AI coding, some of us want to use Herd in worktrees to spin up sites through our favourite apps.

For example I use Conductor (https://conductor.build) which has a setup that provides environment variables for things like the worktree name.

Some idea:

Support .env vars everywhere in the herd.yml, for example in the name property so I could do name: ${SITE_NAME}. This way, my setup scripts would define the environment for their worktree and Herd would automatically create the desired resources (site and services).

Or I could create a herd.conductor.yml file on the fly that contains the Conductor specific environment variables baked in and run it like herd init --file herd.conductor.yml. Again, this would help as we can maintain different .yml files for different agentic platforms and keep the default herd.yml in place for people that just want Herd up and running without the fluff.

Finally, perhaps make the site name optional, and if not given here do not create a Site, only the services (as they sit above sites anyway). That way we don't need to use herd init to create the Site, we can use herd link... later.

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Fantastic product, love using it, just this one little gap is missing to make it truly great for the AI era