v0.87.3: the docs-register sweep

A docs-only release: the docs-register sweep. Every published docs page (the four arms, the
docs index, and the repo README) now conforms to the register standard banked at
docs/internal/docs-register.md.

Changed

• The docs-register sweep corrected two factual errors in the arm indexes, removed marketing
  register, internal plan citations, and private consumer names from public pages, moved git
  vocabulary out of the editor-facing guides, and restructured the tell cadences the standard
  bans. The two front doors (README.md and docs/README.md) were rewritten to the standard's
  front-door register.

No code changes. No consumer action; the bump is drop-in.