feat: add overrideSystemPrompt option for custom modes

[roomote]

This PR attempts to address Issue #11793.

Summary

Adds an overrideSystemPrompt boolean option to ModeConfig that allows custom modes to use their roleDefinition as the entire system prompt body, skipping all standard built-in sections (CAPABILITIES, TOOL USE, MODES, SKILLS, RULES, SYSTEM INFO, OBJECTIVE, MARKDOWN RULES).

Changes

1. packages/types/src/mode.ts - Added overrideSystemPrompt: z.boolean().optional() to modeConfigSchema
2. src/core/prompts/system.ts - When overrideSystemPrompt is true on the resolved mode config, generatePrompt() returns only the roleDefinition + custom instructions (from .roo/rules-*, global settings, and mode-specific instructions)
3. src/core/prompts/__tests__/system-prompt.spec.ts - Added 4 test cases covering: override active, custom instructions still appended, override false, and override unset

Usage

{
  "customModes": [
    {
      "slug": "my-mode",
      "name": "My Mode",
      "roleDefinition": "[Complete custom system prompt]",
      "overrideSystemPrompt": true,
      "groups": ["read", "edit"]
    }
  ]
}

What still works with override enabled

• Tool calling (tools are sent natively, not in the system prompt)
• Custom instructions from .roo/rules-* files and global settings
• Mode switching and tool group restrictions

What is skipped

• All built-in prompt sections (CAPABILITIES, TOOL USE, MODES, SKILLS, RULES, SYSTEM INFO, OBJECTIVE, MARKDOWN RULES)

Backward compatibility

• overrideSystemPrompt defaults to false/undefined, so all existing modes behave exactly as before

Feedback and guidance are welcome.

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