feat(tui): add destructive command guard settings

[Pimpmuckl]

Summary

• add Destructive Command Guard status and management to /codexplusplus
• detect managed, external, repair, update, enabled, and unsupported states without blocking the TUI
• add a once-per-install DCG introduction that is recorded only after its first successful frame render
• preserve staged Codex++ setting changes when entering a DCG management flow
• update the Codex++ fork ledger

Validation

• just test -p codex-tui dcg_views_snapshot
• just test -p codex-tui dcg_nux_persists_globally_and_prepares_once
• just test -p codex-tui welcome_help_first_and_later_startup_snapshot
• cargo insta pending-snapshots --manifest-path tui/Cargo.toml
• just fix -p codex-tui (passes with the pre-existing accounts_tests.rs too-many-arguments warning)
• just fmt
• Review Suite fast signoff: clean (rvw_302da961)

The full codex-tui suite was also exercised during implementation. All feature-specific tests passed; unrelated existing snapshot drift remains in mainline version/workstation-instruction snapshots.

Scope

The final diff is exactly 600 changed lines and keeps fork-owned behavior isolated under codex_plus_plus with minimal upstream seams.