docs: add shared Codex Claude and Cursor onboarding

[jzferrell26]

Summary

Add student-facing project instructions for Codex, Claude Code and Cursor Agent.

• AGENTS.md is the shared source for setup, project boundaries, privacy, media review, external-action approval and verification.
• CLAUDE.md imports the shared guide; a small always-applied Cursor MDC rule points to it.
• README includes the entry-point table, a safe demo starter prompt, loading checks and official documentation links.
• The existing student workflow links to the same guide.

Scope

Five documentation/rule files only. No runtime, dependency, permission configuration or media changes. No personal/global agent setup is bundled or required.

Verification

• npm run check: TypeScript, 32 tests and hygiene pass.
• npm run demo: 1920x1080 H.264 render completed; duration and 30fps verified with ffprobe.
• Static imports, paths, MDC metadata, npm commands and hidden-Unicode checks pass.
• npm audit: zero known vulnerabilities.
• Security review completed before plan-relative quality review; no blocking findings.

Instruction formats were checked against official documentation. Fresh Claude Code/Cursor Agent discovery sessions were not run; README tells students how to verify loading. No live delivery or private-media upload was performed.

Compatibility and rollback

Shared plain-text files work without symlinks or private plugins. Existing runtime behavior is unchanged. Revert this documentation commit to roll back.