fix(init): include codex in global uninstall

[wyf027]

Summary

• Fixes Bug Report: rtk init -g --uninstall cannot work with codex on Ubuntu 20.04 #1450.
• Include Codex RTK artifacts in the default rtk init -g --uninstall cleanup path.
• Reuse the existing Codex uninstall implementation and prefix Codex entries in the combined uninstall summary.
• Update the restart guidance and add a focused unit test for the Codex cleanup aggregation.

Test plan

• cargo fmt --all --check
• cargo test append_codex_uninstall_items -- --nocapture
• cargo test uninstall_codex_at -- --nocapture
• git diff --check
• cargo fmt --all && cargo clippy --all-targets && cargo test
• Manual testing: rtk init -g --uninstall output inspected

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