[harness] Add Cursor harness integration example

[santoshkumarradha]

Parent Epic

Part of #291 — Add Cursor as a Harness Provider

Summary

Create a practical example demonstrating how to use Cursor as a harness provider in AgentField. This should mirror the existing examples/go_harness_demo/ pattern but showcase Cursor-specific capabilities.

Depends On

• [harness] Add Cursor CLI provider to Python SDK #292 (Python SDK CursorProvider)
• [harness] Add Cursor CLI provider to Go SDK #293 (Go SDK CursorProvider)

Proposed Structure

examples/cursor_harness_demo/
├── README.md
├── python/
│   ├── requirements.txt
│   ├── basic_usage.py          # Simple prompt → result
│   ├── with_schema.py          # Structured output via schema
│   └── multi_provider.py       # Same task across cursor + claude-code + codex
└── go/
    ├── go.mod
    └── main.go                 # Go equivalent

What the Example Should Demonstrate

1. Basic execution — Send a prompt, get a result
2. Structured output — Use schema validation (.agentfield_output.json pattern)
3. Session resume — Execute, get session_id, send follow-up with --resume
4. Provider comparison — Run the same task across Cursor, Claude Code, and Codex to show interchangeability
5. Error handling — What happens when Cursor CLI isn't installed, API key missing, timeout

README Should Cover

• Prerequisites (Cursor CLI installation, API key setup)
• How to run each example
• Expected output
• How Cursor fits into the broader AgentField harness ecosystem

Acceptance Criteria

• Working Python examples for basic, schema, and multi-provider usage
• Working Go example for basic usage
• README with setup instructions and expected output
• Examples run successfully with Cursor CLI installed
• Graceful error messages when Cursor CLI not available