Add built-in multi-account round-robin switching for ChatGPT OAuth

[chindris-mihai-alexandru]

Problem

The plugin currently authenticates a single ChatGPT OAuth account at a time. When limits are hit, there is no built-in account pool rotation similar to antigravity-based plugins.

Proposal

• Persist a local account pool (~/.opencode/openai-codex-accounts.json)
• Support account selection strategies:
  • round-robin (default)
  • sticky
• On 429/usage-limit responses, mark account cooldown from response headers and rotate to next available account
• Keep token refresh per selected account

Implementation

I implemented this in PR #113:

• feat: add multi-account round-robin rotation for ChatGPT OAuth #113

Includes tests, docs updates, and installer cleanup support for the account-pool file.