[Usability] Clarify sandbox and approvals indicators in UI — Docs

[github-actions]

Summary

Make it obvious where sandbox mode and approval policy are shown in the TUI and how to change them, reducing uncertainty for new users.

Current vs Expected

• Current: Indicators exist (History panel shows Sandbox: …) and a modal to choose presets, but discovery relies on exploration.
• Expected: Getting Started includes a short section with screenshots and key commands/shortcuts.

Proposed Changes

• Add a subsection to Getting Started explaining: reading indicators, opening the presets modal, effects of Full Auto.
• Cross‑link to docs/config.md for persistent settings.

Acceptance Criteria

• Docs updated with annotated screenshots.
• Footer hints confirm discoverability where appropriate (e.g., Esc edit prev, ? help if implemented).

References

• codex-rs/tui/src/history_cell.rs, codex-rs/tui/src/chatwidget.rs