Add optional .env.example and AGENTS.md setup prompts

[joerucci]

Summary

• Add setup flags to create .env.example during setup, with selectable value modes for blank, non-sensitive, or all example values.
• Add setup flags to optionally add Ghostable guidance to AGENTS.md and persist the managed block through setup.
• Update setup output and JSON payloads to report the new example and agent-instructions actions.
• Expand setup coverage with interactive and non-interactive tests for the new flows.

Testing

• Added unit tests covering example-file generation, prompt handling, JSON output, and opt-out behavior.
• Added unit tests covering AGENTS.md creation, skipping, and existing managed-block detection.
• Not run (not requested).