feat(tui): restyle the question and plan approval prompts with a quiet rail

[edenreich]

Summary

The AskUserQuestion and plan approval prompts share one look now: no box, a thin accent rail on the left, a header chip and dim key hints.

│  Format  1 of 2                              │  Plan  pending approval
│                                              │
│ Which output format should the export use?   │ Restyle the prompts
│                                              │ Steps
│ ❯ JSON (Recommended)                         │ 1. Rework the huh theme
│     structured, easy to parse                │ 2. Rebuild the plan actions
│   YAML                                       │
│     human friendly                           │  Accept   Reject   Approve each step
│   Other…                                     │ ←→ choose · enter confirm · esc reject
│
│ ↑ up • ↓ down • / filter • enter submit

Changes

• Question prompt - the rounded box is replaced by the rail. The header is a chip with an N of M counter (hidden for a single question), the question is bold, and each option's description sits dim on its own line. Selectors use ❯, multi-select uses ✓ / •. The free-text field is titled "Your answer".
• Plan approval - the plan sits behind a rail in its status color with a Plan chip. The selected action is a solid accent pill, the others are dim, and the working keys are listed underneath. This removes a lookup of a theme color that does not exist (selection_bg).
• Shared helpers - RenderRail and RenderChip on the style provider.
• Status lines - shortened to "Waiting for your answer" and "Plan ready for review", since they repeated the key hints.
• Plan approval placeholder - now reads "Plan approval required". The old text advertised h/l, y, n and s, which are not reachable.
• docs/plan-mode.md - matches the new status line and the keys that work.

Testing

• task lint and go test ./internal/presentation/... pass.
• Driven live in tmux against the mock gateway: single select, multi-select, the free-text answer, moving across the plan actions, accepting a plan, and the question prompt at a 50 column width.
• Not checked live: the accepted and rejected plan states, and the light theme.

Not in this PR

• The y / n / s / h / l plan approval keys are still dead. That is a keybinding bug.
• The tool approval box keeps its bordered style.

[inference-gateway-releaser]

🎉 This PR is included in version 0.226.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀