docs(ecosystem): add opencode-qwen-auth plugin

[foxswat]

What does this PR do?

Adds opencode-qwen-auth to the ecosystem plugins list.

This plugin enables Qwen API authentication via OAuth device flow. Features:

• Zero-config OAuth device flow (no API keys needed)
• Multi-account rotation (round-robin/sequential)
• Automatic token refresh
• Rate limit handling (429 detection)
• CLI installer: bunx opencode-qwen-auth install

npm: https://www.npmjs.com/package/opencode-qwen-auth

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