tui: show the cost estimate in the mini/run footer #60653

Follow-up to #59864 (which closes #59861). The experimental["cost-estimate"] experiment currently covers the main TUI sidebar and prompt footer only. In mini/run mode a provider that reports $0 still shows no spend at all.

Current behavior

packages/tui/src/mini/footer.view.tsx renders spend from props.state().usage.cost, which is populated per step from session.step.ended in stream-v2.transport.ts (event.data.cost || undefined). A reported zero is therefore indistinguishable from "cost not reported" and the line disappears.

What it needs

• Expose the experiment to mini: RunTuiConfig in mini/types.ts picks a fixed set of config fields and does not include experimental.
• Session-level totals: state.usage is per-step, not accumulated. The estimate needs the session's tokens/model (an SDK session.get, which the runtime already performs for resume and subagent discovery).
• Tiered models: per-call tier surcharges need the loaded assistant steps, like costEstimate in packages/tui/src/util/session.ts does for the main footer.
• Decide scope: the current session only, or the root family like the main TUI. The family needs listing children; the main TUI gets that for free from the client store.

Acceptance criteria

• With experimental["cost-estimate"] = true and a provider reporting cost: 0, the mini footer shows ≈ $X from catalog prices.
• Positive reported costs keep precedence.
• Sub-cent spend renders as ≈ <$0.01.
• Sessions without catalog pricing omit the estimate instead of under-reporting.
• No change when the experiment is off.