docs(design): the engine gaps left open after the SDK batch

Issues #564 to #618 checked against main at c688fca: six were fixed and are
closed, two are not engine work, and four remain. Their designs, together with
the two gaps the 2026-09-11 record left open, of which one turns out to be
misdiagnosed: the per-target tool declaration exists under
[target.<selector>.xlings.workspace], measured on linux-x86_64.

For review before implementation.

Author: speak-agent