docs: explain how to load agent-shell w/ project-local env vars

[jinnovation]

Closes #258.

[Lenbok]

For those using mise rather than direnv, I'm guessing (since https://github.com/eki3z/mise.el says it was heavily inspired by the envrc package) that it fits into the latter "just works" case?

[xenodium]

If I'm understanding correctly, both envrc.el and direnv.el set process-environment. If so, howcome we need a deferred function for one, but not the other?