docs: prepare OpenAI Build Week submission

[JigSawPT]

Summary

• documents the OpenAI Build Week extension and prior-work boundary
• adds judge testing, Devpost copy, and the owner-only submission checklist
• adds a privacy-safe 98-second demo project with local narration and real Window Swap footage

Validation

• python -m pytest -q — 15 passed
• hyperframes lint — 0 errors, 0 warnings
• hyperframes check — passed
• final MP4: 1920x1080, 30 fps, H.264/AAC, 98.005 s
• worktree and sanitized history scans found no credentials, private paths, private email, retired branding, or private keys

Owner-only before submission

• confirm the qualifying Codex session used GPT-5.6
• run /feedback and add the Session ID
• watch the final MP4 and confirm the narration
• upload it publicly to YouTube
• accept the Devpost rules and submit

This PR intentionally keeps contest materials isolated from main and is not intended to be merged as a general product change.