feat(tui): offer the other backend once the first one works

[plombeer31]

Stacked on #225 → #224 → #223 → #222 → #220.

Why

atomic-agent runs local and cloud side by side and switches between them mid-session. The first run never said so, so an operator who configured one left believing that was the choice they had made — and the second backend is a switch away, not a reinstall.

The screen

  ✓  Cloud model ready

  atomic-agent runs both side by side — local for private or offline work,
  cloud for the heavy turns, switchable mid-session. You have one of the two.

  ›  Set up local models too
     one download, then it runs offline and costs nothing per token

     Skip — take me to the agent
     you can add it later from the menu (ctrl+p)

Mirrored when local came first ("Set up a cloud model too — an API key and a model, about a minute, for the heavy turns").

When it is not shown

decideSecondBackendOffer is a pure function with the whole rule set, and a test per rule:

situation	offer
finished with cloud, no local	local
finished with local, no cloud	cloud
both already configured	none
finished with a custom endpoint	none — that operator has answered the question by running the server
skipped setup	none — they asked to be left alone
already offered once	none

The stamp is written when the offer is shown, not when it is answered: the offer was made either way, and a declined one must not come back on the next launch.

Flow

Accepting re-enters the other branch — the model picker, or the providers wizard — and the flow finishes for real when that lands (by then alreadyProposed is set, so it closes rather than looping). Declining hands over to the agent.

The decision lives in the host effect rather than the reducer, because it reads config; the reducer stays pure.

Tests

propose-second-backend.test.ts — one case per rule above. onboarding-propose-step.test.tsx — both directions of the offer and the cursor.

Full suite: 5196 passed. fs-glob-real and send-message-concurrency fail on main; bootstrap.test.ts flaked under parallel load and passes in isolation (26/26).