Replace the web host submodule with an in-tree import

The web host is referenced as a submodule today. A submodule change is a separate commit in
a separate repository, so a host referenced that way cannot take part in a
single-pull-request check, which is the goal of this epic.

Web goes last: it is referenced from the Makefile, the end-to-end targets and the
playground skills, so it has the widest local-tooling blast radius. Everyone will also be
left with a stale submodule directory.

Depends on: #676, the last of the Android structural work. Deliberately not #677, so
web is not blocked on a scope decision about integrating the core into Android.

Steps

• Remove the submodule reference
• Add the tracked tree at a recorded commit under hosts/web
• Update the Makefile targets, end-to-end targets and skills that reference the old path
• Redirect contributors to this repository

Done when

The web host builds from hosts/web with no submodule, and no reference to the old path
remains.