docs(codex): avoid prefixing shell builtins

[nightcityblade]

Summary

• Clarify that Codex should prefix executable commands, not shell builtins such as cd.
• Document how rtk rewrite places RTK within a compound cd && uv run pytest command.
• Add a regression test for that exact rewrite path and explain how to verify RTK availability.

Fixes #3331.

Test plan

• cargo fmt --all -- --check && cargo clippy --all-targets -- -D warnings && cargo test
  (2,512 unit tests and 64 integration tests passed; 8 unit tests ignored)
• Manual testing: ./target/debug/rtk rewrite 'cd backend && uv run pytest tests/'
  printed cd backend && uv run rtk pytest tests/

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

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