### What problem does this PR solve?
Implement OpenAI chat completions in GO
POST /api/v1/openai/<chat_id>/chat/completions
OpenAI chat cli: internal/development.md
### Type of change
- [x] Refactoring
Implements POST /api/v1/searchbots/ask in Go with streaming SSE,
citations, and think-tag processing. 23 files, 90+ unit tests.
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Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>