[FEATURE]: System prompt mode switcher (Default/Light)

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Problem

The current system prompts are heavy and optimized for frontier models.
This causes issues with:

• Smaller local models hitting context limits
• Token waste for simple tasks
• Users who want minimal prompting for specific workflows
• Outdated instructions that don't match current model capabilities (some prompts unchanged for 7+ months)
• Conflicts between default prompts and user's AGENTS.md instructions
• Overspecification that degrades reasoning in high-capability models #10927

Proposal

Add a system prompt mode setting with initial options:

• Default: current per-model prompts (anthropic.txt, gemini.txt, etc.) — no changes
• Light: minimal prompt with only essential info: tools, basic safety rules, environment

The current proposal covers several things at once:

• Non-breaking: Default behavior unchanged
• Extensible: Opens path for future custom mode where users provide their own prompt
• Addresses multiple issues without requiring prompt rewrites

Configuration

• Via settings menu (Ctrl+P → Settings)
• Via opencode.json:

{
   "prompt": {
     "mode": "light"
   }
 }

Related issues

• #7101
• #10927
• #11995
• #13605
• #14398