fix(integration): sync file-based credentials (~/.gemini/oauth_creds.json) on account switch

When switching accounts in Antigravity-Manager, tokens are written to system keyring via secret-tool (on Linux). However, when running the CLI tool (agy) inside an SSH session, agy detects SSH and falls back to file-based token storage (~/.gemini/oauth_creds.json), bypassing system keyring lookup.

This commit syncs account credentials to ~/.gemini/oauth_creds.json and ~/.gemini/google_accounts.json alongside system keyring writes. This ensures account switches take effect seamlessly in SSH sessions, container environments, and headless environments where D-Bus/Keyring is unavailable.