feat: add interactive multi-account pool and agy config isolation

[ganimalqudhaifi]

This PR introduces a robust multi-account management system with an interactive Terminal User Interface (TUI).

Key Features:

• Multi-Account Pool: Support saving multiple Google accounts with automatic rotation and state tracking.
• TUI Auth Menu: Interactive CLI menu to check quotas, switch active accounts, or delete accounts natively.
• Auto-Resolve Emails: Automatically fetches and maps Google account emails upon authentication/token refresh.
• Config Isolation: Refactored config paths to store accounts cleanly in ~/.config/opencode/agy/accounts.json and moves temporary caches to the system's tmpdir.
• AgySwitch Tool: Exposes /agyswitch command for seamless context switching.
• Documentation: Updated README to reflect the new persistence paths and tools.