init --no-template, ext ls and init setup still read only environments.toml #622

These paths still read only environments.toml, so they do nothing or misbehave on scaffold.yml v2 projects:

• init --no-template: disable_clients_in_env_toml (instantiate.rs#L156) finds no environments.toml and returns Ok(()). build --build-clients then still deploys and writes clients into app-lib/clients, a directory init just removed. v2 needs a way to skip clients, or this function should go. Fixed in Allow clients: false per contract in scaffold.yml #614.
• init Docker warning: fires only when environments.toml sets run_locally (setup.rs#L57), so v2 projects get no warning before the deploy fails. Fixed in fix: scaffold.yml v2 fast-follows for the 1.0 preview #632.
• init extension install: ensure_extensions_installed (setup.rs#L206) never sees v2 extensions:.
• ext ls: lists extensions from environments.toml only, and takes a legacy [ENV] argument (ext/ls.rs#L40).
• Help and messages: build/watch --help still describe environments.toml and STELLAR_SCAFFOLD_ENV; generate contract tells users to edit environments.toml.