feat(demo): add System prompt tab to the Use-it guide

[mayeedwin]

Adds a dedicated System prompt tab to the demo's "Use it" guide dialog.

Tabs are now: How it works · System prompt · React · Angular.

The new tab shows:

• A one-line intro to getSystemPromptInstruction() from @conversed/core
• The copyable snippet:

  import { getSystemPromptInstruction } from '@conversed/core';
  
  const systemPrompt = `
  You are an AI assistant.
  ${getSystemPromptInstruction()}
  `;

• The "renders content, not conversations" note — Conversed never owns roles/avatars/bubbles; you drop <ConversedContent> into your own assistant bubble and hand it blocks.

Verified: pnpm --filter @conversed/demo-react build and lint pass; getSystemPromptInstruction confirmed as a real export.

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.