* fix(auth): prioritize round-robin over lastGood for multi-account rotation
When multiple OAuth accounts are configured, the round-robin rotation was
not working because lastGood was always prioritized, defeating the sort by
lastUsed.
Changes:
- Remove lastGood prioritization in resolveAuthProfileOrder
- Always apply orderProfilesByMode (sorts by lastUsed, oldest first)
- Only respect configuredOrder when explicitly set in config
- preferredProfile still takes priority for explicit user choice
Tested with 2 Google Antigravity accounts - verified alternating usage.
Follow-up to PR #269.
* style: fix formatting
Adds usage tracking to auth profiles for automatic rotation:
- ProfileUsageStats type with lastUsed, cooldownUntil, errorCount
- markAuthProfileUsed(): tracks successful usage, resets errors
- markAuthProfileCooldown(): applies exponential backoff (1/5/25/60min)
- isProfileInCooldown(): checks if profile should be skipped
- orderProfilesByMode(): now sorts by lastUsed (oldest first)
On auth/rate-limit failures, profiles are marked for cooldown before
rotation. On success, usage is recorded for round-robin ordering.
This enables automatic load distribution across multiple accounts
(e.g., Antigravity 5-hour rate limit windows).
