test(repository): document cross-platform gui workflow

[StudentWeis]

Summary

Add a repository-local Codex skill for repeatable Ropy GUI end-to-end testing across macOS, Windows, and Linux. macOS runs locally; Windows and Linux run in isolated AgentBay virtual desktops.

Linked Issue

Closes #159

Changes

• Add a shared packaged-app smoke scenario for process, hidden startup, hotkey, clipboard capture, search, confirmation, and paste.
• Add platform references for safe local macOS execution, Windows AgentBay deployment, and Linux AgentBay X11/architecture/runtime validation.
• Add cost, credential, evidence, failure-classification, and guaranteed virtual-session cleanup guardrails.
• Add cross-platform skill UI metadata.

Testing

• scripts/precheck.sh passes locally (455 tests)
• Skill passes quick_validate.py
• Linux shell example passes bash -n
• Platform facts checked against release assets and Ropy source configuration

Self-Check

• PR title follows Conventional Commits
• No hardcoded user-facing strings — documentation-only change
• Errors defined with thiserror — no error types added
• No unrelated changes mixed in