docs: regroup the README so the human half comes first

The first thing a visitor saw was the problem statement, then thirty lines
of mechanism before any evidence the thing works. Six sections now sit above
the fold in decision order: the /clear proof, the problem, the three files,
why it works, the numbers, install. Everything technical moves below a new
Reference index.

Two jump indexes: a five-link one under the badges for the human half, and a
table under Reference covering the technical half.

New headline leads on the differentiator rather than the symptom. A hook
that fires every turn is not the same as a prompt the model may choose to
follow, and that is the whole argument for this skill. The previous tagline
is kept underneath.

Nothing removed. Verified section by section: 22 blocks each used exactly
once, every one byte-identical, 0 of 462 substantive lines lost, 0 headings
lost. Also repairs #faq, which stopped resolving when the FAQ became a
collapsed details block, and gives Releases and Community real anchors.