feat(onboarding): drop first-run users into a modified /resume picker with a top onboarding banner and Start-new-session option

Replaces the Yes/No 'Continue where you left off' prompt and the 10s auto-select-latest behavior. When external Codex/Claude Code transcripts exist, onboarding opens the resume picker directly: the top ~20% shows a formatted onboarding prompt plus a selectable 'Start a new session' row, with the session list below for resuming.