Adds repo:admin to the device-flow login DEFAULT_SCOPES so a normal anvil auth login token can use the shipped anvil ci set-secret/delete-secret/list-secrets commands (the server gates those on repo:admin, matched exactly — no scope hierarchy).

The old comment conflated the ORG-level admin:* scopes (correctly excluded) with the REPO-level repo:admin, dropping the latter. One-line scope addition + comment fix; cargo build clean.

Diagnosed via a source audit of anvil (server) + anvil_cli. Closes #25.

Follow-up (not in this PR): a --scope flag on auth login to override the default set.