The three research notes in docs/research/ already use sentence case headings,
but AGENTS.md, README.md, the descriptor doc, and the auto-update plan and spec
still carry title case from before that convention settled. Bring them in line
so the same rule holds across the repo.

Also drops the two em dashes in AGENTS.md, rewriting the sentence rather than
trading them for parentheses, and replaces the curly quotes in the 0.49.1
parity note with straight ones.

Extend the em dash and en dash rejection from main.qml to the whole applet
surface. The rule follows the visible text, not the file it lives in: a dash in
an extracted component reads the same in the popup, and the check as written
would not have seen it. AGENTS.md already asks for surface-wide assertions; this
check predated that guidance.