[codex] Add desktop first-run quickstart docs

[spaceshipmike]

Summary

• Add a concise desktop first-run quickstart to the public README.
• Document the app first-run user flow before developer/release notes.
• Update the public-readiness plan with docs-slice proof and remove the stale folder-picker TODO.

Verification

• git diff --check
• git diff --cached --check

No build/tests run: docs-only changes.