fix: remove unsupported AI providers

[GioiaZheng]

Summary

• remove OpenAI/custom from the Web AI provider selector
• update the config schema so it only advertises the currently supported Anthropic provider
• update README requirements and FAQ to avoid promising unsupported provider routing
• rebuild the tracked frontend bundle

Tests

• python -c "import json; json.load(open('src/bosshunter/web/config_schema.json', encoding='utf-8')); print('ok')"
• npm run build
• git diff --check

Fixes #2

[powerycy]

Reviewed the AI provider UI/schema cleanup. Removing OpenAI/custom from the advertised options aligns the UI with the current Anthropic-only backend and avoids misleading users until provider routing is implemented.