Fix Codex global RTK reference path on macOS

[jiemojiemo]

Description:

Summary

This PR fixes a Codex global init issue on macOS where rtk init -g --codex wrote @RTK.md into ~/.codex/AGENTS.md, but Codex resolved that reference relative to the current project instead of ~/.codex. As a result, Codex could fail to find the global RTK.md when opened in another repository.

What changed

• Update Codex global init to write the absolute ~/.codex/RTK.md path into ~/.codex/AGENTS.md
• Keep local Codex init behavior unchanged and continue using the project-local @RTK.md reference
• Make AGENTS.md patching migrate legacy global @RTK.md references to the new absolute-path format
• Make Codex uninstall and config detection handle both legacy relative references and the new absolute reference
• Update Codex documentation to describe the correct global behavior

Tests

Added and updated unit tests covering:

• adding the RTK reference only once
• creating a missing AGENTS.md
• migrating inline RTK blocks
• migrating a legacy global @RTK.md reference to an absolute path
• removing the absolute-path reference during Codex uninstall
• preserving uninstall idempotency

Validated with:

cargo test patch_agents_md -- --nocapture
cargo test uninstall_codex_at -- --nocapture

Why this matters

Codex global instructions should remain valid no matter which project is opened. Writing an absolute path for global Codex setup makes the configuration stable across repositories, while preserving the existing local-project workflow.

[CLAassistant]

All committers have signed the CLA.

[Valmonzo]

Thx mate , wanted to do it this weekend but u are so reactive ❤️