Summary

goat provider login and goat search login used to store credentials under a fixed default account when --account was omitted, silently overwriting an existing account. This adds a shared resolve_account flow: an explicit --account name updates that slot; the first account keeps the default name; and when accounts already exist without a name, interactive callers pick an existing account (update) or add a new one (with confirm on collision) while non-interactive callers error and point to --account.

Verification

• cargo fmt --all --check — passed
• cargo clippy --workspace --all-targets -- -D warnings — passed
• cargo test --workspace — passed