Register daytona/sandbox when provision emits credentials

[snowmead]

Blocker

daytona/sandbox is HELD. Link needs Auth0 browser flow; after link + top-up plan, the resource stays pending with no credential env vars (discover hangs).

Unblock

1. Get a sandbox to complete with env outputs from Stripe Projects.
2. Declare pub mod daytona + registry row.
3. discover-apply, drop EXCL, restore PROVIDERS.md.

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

Related: #90