Register twilio/email for US-region Stripe Projects accounts

[snowmead]

Blocker

twilio/email is HELD. Link/add fails for non-US accounts:

not_in_country / Currently, only US-based accounts are supported

Unblock

1. Provision from a US-based Twilio/Projects account (or wait until Twilio lifts the region gate).
2. Declare pub mod twilio + registry row.
3. discover-apply, drop EXCL, restore PROVIDERS.md.

See docs/ADDING-A-PROVIDER.md § External pin blockers.

Related: #90