Add custom prompt configuration for Issue Responder

[cdnsteve]

Summary

Adds the ability to customize Issue Responder behavior per-project using a JSON configuration file.

New file location: .sugar/prompts/issue_responder.json

Features

• Custom instructions: Project-specific system prompt additions
• Persona configuration: Define role, goal, and expertise
• Guidelines: List of things Sugar SHOULD do
• Constraints: List of things Sugar should NOT do

Example Configuration

{
  "name": "my-project-responder",
  "description": "Custom issue responder for my project",
  "persona": {
    "role": "Senior Developer",
    "goal": "Help users resolve issues quickly",
    "expertise": ["Python", "TypeScript", "REST APIs"]
  },
  "instructions": "You are a helpful assistant for this project...",
  "guidelines": [
    "Always search the codebase before responding",
    "Include relevant file paths"
  ],
  "constraints": [
    "Never share API keys or secrets",
    "Don't promise release dates"
  ]
}

Changes

• sugar/config/prompt_config.py - New config loader with PromptConfig and PersonaConfig classes
• sugar/profiles/issue_responder.py - Load and use custom prompts
• examples/issue_responder_prompt.json - Example configuration
• tests/test_prompt_config.py - 24 tests (96% coverage)
• docs/issue-responder.md - Full documentation for custom prompts
• README.md - Updated with custom prompt feature

How It Works

1. If .sugar/prompts/issue_responder.json exists, it's loaded
2. Config is validated (instructions field required)
3. Custom prompt sections are appended to base system prompt
4. If file missing or invalid, default behavior unchanged