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

[dzianisv]

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

[dzianisv]

Root cause: no always-visible UI element for switching build/plan modes; previous agent created wrong feature (shell toggle) on an unmerged branch.

Shipped in commits cc73c6a + 3182f65 on dev.

Changes:

• packages/app/src/components/prompt-input.tsx — showBuildPlanToggle memo + Build/Plan segmented toggle in v2 toolbar before model selector
• packages/app/e2e/utils/mock-server.ts — optional agents? field in MockServerConfig
• packages/app/e2e/regression/prompt-mode-switch.spec.ts — 7 regression tests

Prod verifier: 7/7 e2e tests pass (see .tasks/237/verify.md). PROD: pass.

Review fixes (3182f65): showBuildPlanToggle now checks options (filtered) not available (raw), preventing hidden agents from rendering a mis-selecting toggle.