docs: prove ChatGPT subscription harness end to end

[munirad7s]

Summary

• replaces the expired 2026-08-01 quota blocker with the verified 2026-08-13 end-to-end result
• documents the ChatGPT OAuth chain, isolated CODEX_HOME, and API-key-negative control
• records the signed #build reply and the Windows CEST/NIP-98 clock trap
• updates stale cross-references that still described buzz#18 as blocked

Verification

• codex login status: Logged in using ChatGPT
• codex-acp 1.1.7 initialized ten agents and exposed the chat-gpt auth method
• OPENAI_API_KEY was removed from the proof harness child environment
• dispatcher reply CHATGPT_ABO_OK exists as event 409e4fa2391a6da72a0cba9a438ef8c2ce1a37bbc820493ba4a9242f00eef470 with the expected root reference
• Buzz Desktop restarted the dispatcher under app management; relay connection, #build subscription, and ten-agent pool are healthy
• git diff --check passes

Closes #18

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