docs(changelog): record the skill-description budget round, and credit the measurement behind it

Ten commits landed the round and none of them said in the CHANGELOG what it was or why it happened.
It happened because Jan Mangs measured it: 18,455 characters authored, 6,300 retained by Codex,
12,155 discarded, fifteen descriptions cut mid-token — and, more usefully than the numbers, the
diagnosis that the failure is semantic rather than literal, because related skills lose the clauses
that distinguish them.

That reading is what turned the task from "compress eighteen descriptions" into "find the set of
skills whose boundaries survive 350 characters", which is why a skill was folded away rather than
merely trimmed. Recorded here with the pre-registration, the two amendments made before measurement,
and the REJECT on the registered band.

Co-Authored-By: Jan Mangs <6362479+jmangs@users.noreply.github.com>
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>