Define authenticated transport policy for remote extension allowlists

**What problem would this solve?**

Remote extension allowlists are an administrator policy input, but Goose currently accepts any configured URL and follows redirects without defining an authenticated-transport requirement. Existing deployments may rely on plaintext HTTP, especially for local services, so changing this needs an explicit compatibility policy.

**What would a good outcome look like?**

Remote allowlist content has a documented transport-authentication rule that is enforced for the initial URL and every redirect. Administrators get a clear error for disallowed configurations and a narrowly defined migration path where needed.

**Possible approaches**

- Require HTTPS for all remote allowlists and reject downgrade redirects.
- Permit plaintext HTTP only for loopback addresses.
- Offer an explicit insecure compatibility override with prominent diagnostics and documentation.

The decision should cover redirects, local certificate deployments, startup failure behavior, and existing managed installations.

**Additional context**

- [x] I have verified this does not duplicate an existing feature request

Do not begin implementation until the issue reaches **Ready** on the [Goose Issues board](https://github.com/orgs/aaif-goose/projects/1).