[Flow] Document first‑run onboarding and trust flow — TUI

[github-actions]

Summary

New users experience a trust/onboarding sequence (approvals/sandbox, workspace trust). Provide a step‑by‑step doc with screenshots and expected states to reduce confusion and support troubleshooting.

Current vs Expected

• Current: Onboarding code exists (tui/src/onboarding/*) and some behaviors are noted in docs, but no end‑to‑end walkthrough.
• Expected: A concise “First Run” guide with visuals showing prompts, options, and where settings surface in UI.

Proposed Changes

• Add a “First Run” doc under docs/ with screenshots of each step.
• Clarify where sandbox/approval policies are displayed (History panel summary) and how to change them.
• Link from Getting Started and README.

Acceptance Criteria

• New doc with at least: trust directory step, approvals/sandbox presets picker, status indicators.
• README and docs/getting-started.md link added.
• Troubleshooting section for common blockers (permissions, terminal quirks).

References

• codex-rs/tui/src/onboarding/*, codex-rs/tui/src/history_cell.rs