docs: ONBOARDING_FLOW_DIAGRAM.md is a 0-byte placeholder

[phertyameen]

Summary

ONBOARDING_FLOW_DIAGRAM.md sits at the repo root next to ONBOARDING_QUICKSTART.md and ONBOARDING_IMPLEMENTATION_SUMMARY.md, but contains zero bytes - anyone following the onboarding documentation chain hits a dead reference.

Options

1. Fill it: add a Mermaid sequence diagram of the onboarding flow (landing -> IQ survey -> account/guest selection -> first puzzle); GitHub renders Mermaid natively.
2. Delete it: remove the file and any links pointing at it, folding anything useful into ONBOARDING_QUICKSTART.md.

Either outcome is fine; an empty file is not.

Acceptance criteria

• No 0-byte documentation files remain at the repo root.
• Links/references updated accordingly.