fix(v2): persist AGY OAuth auth atomically

[marcelokarval]

Summary

Follow-up for #132 and the active OpenCode V2 compatibility work in #107.

• replace direct auth.json writes with an exclusive same-directory 0600 temporary file followed by atomic rename;
• retain other providers while updating google-agy;
• avoid replacing corrupt existing auth stores;
• clean up only a temporary file opened by this invocation;
• return a clear OAuth-login failure if persistence fails, while allowing an already refreshed request to continue with a sanitized warning.

Validation

• npm test -- --run test/plugin/v2-storage.test.ts test/plugin/v2-session.test.ts test/plugin/v2.test.ts — 24 passed
• npm run test:coverage — 486 passed
• npm run typecheck
• npm run build
• npm run smoke:node-import
• git diff --check

Scope

This PR intentionally targets tone/feat/opencode-v2-dual-compat (the head of #107), rather than main, so it is a dependent focused patch rather than a competing V2 implementation. It contains no credentials, auth records, provider responses, or production configuration.

Relates to #132.

[anthonyhaussman]

Thank you for the contribution! The fix has been incorporated and published to npm under @anthonyhaussman/[email protected] (alpha dist-tag).