feat: modernize OpenAI integration with Responses API

[AmirMotefaker]

Closes #3

Summary

Modernizes the repository's supported OpenAI integration to the 2026 Responses API while preserving historical notebooks.

API contract

• OpenAI Python SDK: 2.45.0+
• Responses API: client.responses.create
• Default model: gpt-5.5
• Secret source: OPENAI_API_KEY
• Conversation state: previous_response_id

Validation

• Local Python compile: skipped: no usable local Python; GitHub Actions is authoritative.
• Local modernization contract checks: passed.
• git diff --cached --check: passed.
• GitHub Actions: Python 3.10 + 3.12, dependency install, compile, offline tests, secret scan, legacy API guard.
• No live/paid OpenAI API call is made by CI.

Evidence

• evidence/phase5-openai-modernization.md
• Head SHA: c133d63

Published file SHA-256

• ChatGPT_Web_Application.py — 02A4B13A94F342407AC383220ABEC7BA7327C7735DF8D6D539B9448AF3436BED
• openai_service.py — 003972CEB7C21C7CE4C1960F16F0EE053BDC9F3B7DD3BD4DA2AA6B8B82D3E12B
• requirements.txt — 5F00E2B0612FF88F48B60E2C38B7300C47B6248C3404DF6205F4AB23A92912E7
• .gitignore — BBC6534340039E3E636ADF5A960CAFE7D0E69ABB6EBECFF9E1DBC1F6226F308F
• tests/test_openai_service.py — 1EF61651F84747CC18916E83EBE2D9F6E4009BB507E4C8BEB83B6B89B60C02E9
• .github/workflows/python-modernization.yml — 22BBF9F7EC140AAFC5349CBA73C71FF74B137533FEE438978AC8C1B3748F346C
• evidence/phase5-openai-modernization.md — 0CEDAF71818A1A0B52CDB4D5116474A6C72E0790A83921F9482AD2FC67BE925A
• README.md — BC4C3DF237E0614C0811878EE0B42E0CF6CEDA1DAD6FBB2F393B4405C7CFF52C