feat: implement Google Agy quota provider and companion auth integration

[anthonyhaussman]

Summary

Introduces the google-agy quota provider, enabling the system to fetch and display usage data from the Google Agy Code Assist API.

# Current returns:
> node dist/bin/opencode-quota.js show --provider google-agy
[Google Agy]
Claude and GPT models                           4d
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Gemini Models                                   3d
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Linked Issue

#125

OpenCode Validation

• Current production released OpenCode version tested: 1.17.4
• Why this version is relevant to the fix:

Quality Checklist

• I ran pnpm run typecheck
• I ran pnpm test
• I ran pnpm run build
• This is the smallest safe root-cause fix (no unnecessary hook/output mutation logic)
• I preserved behavioral invariants and updated/added boundary tests as needed
• I updated docs for user-facing workflow/command/config changes (README.md and CONTRIBUTING.md when applicable)
• For new API-key/token providers, I started from contributing/provider-template/ or explained why the template does not apply
• For provider setup/auth wording changes, I checked the relevant dummy .ts template in contributing/provider-template/ and verified README.md against src/lib/provider-metadata.ts (authentication/authFallbacks) and provider auth resolver/diagnostics behavior

[slkiser]

Will adopt after I start and review the README.

I’m not ready to fully kill off opencode-antigravity-auth, but Google is moving fast here and that Noe's plugin is not updating at the same pace. So opencode-agy-auth will briefly share the spotlight after some warnings via announcements.