Register blaxel/agent-drive when private preview opens

[snowmead]

Blocker

blaxel/agent-drive is HELD (agent_drive.rs undeclared; blaxel/sandbox stays registered). Provision returns:

402 private preview — join waitlist (Blaxel Typeform)

Unblock

1. Gain Blaxel agent-drive access / waitlist approval.
2. Add pub mod agent_drive in blaxel/mod.rs + register_providers! row.
3. mise run discover blaxel/agent-drive -- --json --dir fixtures/smoke/cloudflare | mise run discover-apply
4. Drop blaxel/agent-drive from EXCL.
5. Restore the row in PROVIDERS.md.

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

Related: #90