Re-add iTerm2 shell integration for zsh

[jdevera]

Context

iTerm2 shell integration was only wired into the bash config (external target .bash.d/200_tool_iterm2_shell_integration.sh, macOS-only). The bash decommission (92fe313) removed the external entirely, so iTerm2 features that depend on it (command marks, badges, it2* utilities awareness, etc.) are no longer loaded anywhere.

Task

If still wanted, re-add it for zsh:

• New external (or restore iterm2_shell_integration.toml) guarded by is_macos, targeting .config/zsh/zsh.d/, e.g. 200_tool_iterm2_shell_integration.zsh, pulling the zsh variant: https://raw.githubusercontent.com/gnachman/iTerm2/master/Resources/shell_integration/iterm2_shell_integration.zsh
• Previous refreshPeriod was 730h.
• Consider guarding the source with a $TERM_PROGRAM == iTerm.app check (or rely on the script's own no-op behavior) so it stays inert in Ghostty/other terminals.

Alternatively, decide it was never missed and close this.