feat: novel prompting paradigm with 6-fold system prompt

[philipph-askui]

Summary:
introduces a comprehensive system prompting framework for the Vision Agent's act() command, nudging users to write better system prompts and hence to customize agent behavior for their specific applications and UI contexts.

Key Changes:

• feat: introduces a Five-part prompt structure: System Capabilities, Device Information, UI Information, Report Format, Additional Rules
• feat; adds ActSystemPrompt Structure to reflect new prompt structure
• feat: adds prompt factory functions for faster prompt creation
• feat: adds default prompt parts for common use cases (Desktop Device, Android Device, Md Report)
• chore!: moves existing system prompts from askui.prompts.system to askui.prompts.act_prompts as they are only meant for use with the act command
• chore!: moves existing get prompts from askui.models.shared.prompts to askui.prompts.get_prompts