fix: uninstall Codex artifacts in global cleanup

[will-r-wang]

Summary

• Make rtk init -g --uninstall clean up global Codex and Gemini artifacts in addition to the existing global RTK uninstall flow.
• Add a regression test covering global Codex uninstall behavior and keep targeted --codex --uninstall / --gemini --uninstall paths intact.
• Clarify CLI/help text and installation docs so uninstall behavior matches what the command actually does.

Test plan

• cargo fmt --all && cargo clippy --all-targets && cargo test
  • test result: ok. 1607 passed; 0 failed; 6 ignored; 0 measured; 0 filtered out; finished in 0.97s
• cargo test uninstall -- --nocapture
• Manual CLI smoke test: global Codex install + rtk init -g --uninstall with temp RTK_CLAUDE_DIR / CODEX_HOME, verified RTK.md and the AGENTS.md reference were removed
• Manual CLI smoke test: global Gemini install + rtk init -g --uninstall with temp HOME, verified .gemini/GEMINI.md and .gemini/hooks/rtk-gemini.sh were removed

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

[pszymkowiak]

[w] wshm · Automated triage by AI

📊 Automated PR Analysis

🐛 Type	bug-fix
🟢 Risk	low

Summary

This PR makes rtk init -g --uninstall clean up global Codex and Gemini artifacts in addition to Claude Code artifacts, ensuring the catch-all global uninstall removes everything. It also adds regression tests for global Codex uninstall behavior and updates CLI help text and installation docs to reflect the expanded uninstall scope.

Review Checklist

• Tests present
• Breaking change
• Docs updated

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Analyzed automatically by wshm · This is an automated analysis, not a human review.