Update the TUI onboarding to include a note about allowing Ei on Codex if it is detected

[Flare576]

Context

Discovered while porting Ei's WHO/MEMORY hooks to Codex (sisyphus/omp-hook-dedup). Verified live against the real codex-cli 0.142.3 binary: Codex requires an explicit trust review before any non-managed command hook runs at all, even with a perfectly-formed hooks.json already in place (developers.openai.com/codex/hooks — "Before a non-managed command hook can run, Codex requires you to review and trust the exact hook definition... new or changed hooks are marked for review and skipped until trusted").

A live codex exec smoke test against Ei's installed hooks confirmed this directly: the hook silently fired nothing until the invocation used --dangerously-bypass-hook-trust. For a real interactive user, the equivalent unblock is running /hooks once inside Codex to review and trust Ei's two hooks.

src/cli/install.ts's installCodexHooks() already prints a console message pointing at this (Use /hooks in Codex to review/trust the hooks if prompted...), but that message only appears if the user runs ei --install from a terminal and reads its output. The TUI's onboarding flow (tui/src/components/OnboardingOverlay.tsx) already detects Codex (detection().codex, line ~401) and offers to enable the integration, but has no equivalent note — unlike Cursor, which already has exactly this kind of harness-specific caveat:

<Show when={detection().cursor}>
  <text> </text>
  <text fg="#b58900">
    {"Cursor: enabling this installs a hook that injects your recent Ei\n" +
      "context into Cursor's prompts on every request — which may be sent\n" +
      "to Cursor's configured model backend."}
  </text>
</Show>

Without an equivalent for Codex, a user who enables the Codex integration during onboarding gets a hook installed that will silently do nothing until they separately discover and run /hooks inside Codex — no error, no warning inside the TUI itself, just an integration that appears enabled but never actually injects anything.

Proposal

Add a <Show when={detection().codex}> block alongside the existing Cursor one (tui/src/components/OnboardingOverlay.tsx, right after line ~419), following the same style, explaining that Codex requires a one-time /hooks review-and-trust step inside Codex itself before the installed hooks will do anything.

Acceptance Criteria

• Onboarding shows a Codex-specific note (matching the Cursor note's placement/styling) when detection().codex is true and the user is offered the integration.
• The note names the concrete required action: run /hooks inside Codex once to review and trust Ei's hooks, or they will not fire.
• src/cli/install.ts's existing console message (non-TUI install path) is left as-is — this issue is scoped to the TUI onboarding surface specifically.