fix(init): install Codex global instructions in CODEX_HOME

[michaelschleiss]

Summary

Make rtk init -g --codex respect CODEX_HOME when it is set, instead of always writing Codex global files to ~/.codex.

Problem

Codex can use a custom config home via CODEX_HOME, but the global Codex init flow still resolved its install location as ~/.codex. In environments where Codex is configured under a different home, rtk init -g --codex writes AGENTS.md and RTK.md to the wrong directory.

Fix

Resolve the Codex global config directory from CODEX_HOME first and fall back to ~/.codex only when it is unset. Keep uninstall and config inspection aligned with the absolute Codex RTK.md reference used by global installs, and update the hook docs and changelog to match the new behavior.

Test plan

• cargo fmt --all && cargo clippy --all-targets && cargo test
• Manual testing: rtk init -g --codex, rtk init -g --codex --uninstall, and rtk init --codex output inspected

Important: All PRs must target the develop branch (not master).
See CONTRIBUTING.md for details.

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