feat: add optional OpenAI multi-account quota support

[christian-taillon]

Summary

Adds optional, read-only OpenAI multi-account quota discovery for users already using oc-codex-multi-auth.

• With no multi-account store, the existing native OpenAI path is unchanged and no extra account work runs.
• With multiple accounts, cached access tokens are queried independently and displayed with provider-reported plan labels such as Business, Plus, and Plus #2.
• Stable internal source IDs preserve per-subscription accounting without exposing role or account IDs in the UI.
• The companion auth plugin retains ownership of token refresh and persistence; this plugin does not refresh, exchange, or write credentials.

Linked Issue

No linked issue. This is a focused compatibility addition for multi-subscription OpenAI/Codex setups.

OpenCode Validation

• Current production released OpenCode version tested: 1.18.21
• Why this version is relevant: it validates the plugin's server/TUI integration against the current OpenCode runtime.

Quality Checklist

• I ran pnpm run typecheck
• I ran pnpm run build
• I ran pnpm test
• This change is focused and avoids unrelated behavior changes
• I updated or added tests when behavior changed
• I updated docs when user-facing workflow, command, or config behavior changed
• For provider changes, I followed Provider Changes, or this does not apply