Align project with TecCoach direction

[danieldiazde]

Summary

• Make PROJECT.md the source of truth for TecCoach
• Remove the FastAPI backend and old generic Gemini /api/interpret scaffold
• Update README, architecture docs, demo docs, prompts, env templates, CI, and agent instructions for the Next.js + Supabase + Gemini direction
• Replace the old prompt UI with a TecCoach app shell

Verification

• pnpm check
• npm run build:frontend

Notes

main is currently locked, so this PR is ready for owner approval/merge instead of direct merge.