Not a fourth source tree: containers/ becomes .modal/

This file names three source trees and then a `containers/` appeared beside
them, reading as a fourth. It is not one. It is the recipe for building the
others somewhere with more memory than this machine -- CI config that lives
here the way `.github/` would, and it can hide like it.

Nothing but the name changes. `.modal/` sits one level under the root just
as `containers/` did, so container.py still finds _loader.py beside its
parent. `scripts/deploy` looks in both, and new-container writes `.modal/`
by default now.

Also drops three .pyc files a previous `git add` swept in, and ignores
__pycache__ so the next import does not put them back. tools/ still has one
tracked from before; left alone rather than folded into an unrelated move.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>