When a tmux window is created manually with Command-T and its profile
has "Use custom title for new tabs using this profile" enabled, apply
that title. Previously the custom title was applied only to non-tmux
tabs, so new tmux tabs fell back to the tmux window/session name.

The title is an interpolated-string template, so it is evaluated
against the tab's scope up front and the concrete result is used. For a
tmux tab, setting a title override renames the tmux window (shared with
every attached client), so this ensures the server-side name matches
what iTerm displays rather than the raw template text. Only manually
opened windows are affected, so attaching to an existing session no
longer clobbers window names, and a blank result is ignored so the tmux
window is never renamed to an empty string.

Fixes iTerm2 GitLab issue 12945.