Problem: after apply_template writes a file's content to the buffer,
cursor positioning was left entirely to the user's setup_code hook,
forcing everyone to reimplement the same placeholder-stripping logic.
Solution: add an optional templates.cursor_marker config key. When set,
apply_template scans the written lines for the marker, strips it, and
positions the cursor there via bufwinid so it works in both the
provisional and existing-file paths.