feat(app): build/plan mode toggle in v2 composer toolbar #237

Problem

The v2 webapp composer has no visible way to switch between build mode (default: can edit files) and plan mode (read-only planning). Users must know the agent picker dropdown exists and that it's gated behind the `showCustomAgents` setting. The mode switch is effectively invisible.

Goal

A compact, always-visible Build / Plan segmented toggle sits in the v2 toolbar row, below the textarea, before the model and reasoning selectors. Clicking switches the active agent. Active mode shows elevated background; inactive shows muted text.

Acceptance Criteria

• Toggle renders in v2 layout (`newLayoutDesigns: true`) when both build + plan agents are available
• Toggle is absent when only one agent exists
• Toggle is absent in v1 (legacy) layout
• Clicking Plan sets `aria-pressed=true` on Plan button, `false` on Build
• Clicking Build reverses
• Toggle appears before `[data-action="prompt-model"]` in DOM order
• Toggle is hidden in shell mode
• E2e regression tests cover all above cases

Implementation

Commit `cc73c6a68` on `dev` — `packages/app/src/components/prompt-input.tsx`
Tests: `packages/app/e2e/regression/prompt-mode-switch.spec.ts`

Agent-owned