[codex] Fix Ghostty shell startup errors

[mcclayton]

Summary

• Silence installer color initialization when TERM has no local terminfo entry, with a Ghostty fallback to xterm-256color.
• Make splash screen width detection resilient to failed tput calls and quote rainbowtext output so lolcat flags are not treated as printf options.
• Guard the splash clear path so missing Ghostty terminfo cannot emit '''xterm-ghostty''': unknown terminal type.
• Load asdf from interactive zsh startup files and avoid running Bash-only bind commands when .bashrc is sourced from zsh.
• Add shell startup regression coverage for TERM=xterm-ghostty, including a missing-fallback config case.

Validation

• bash ./ci/test_shell_startup.sh
• bash ./ci/checks.sh
• git diff --check