docs(readme): expand stub README to full structure (#26)

Closes Row-2 Phase 1 of the 2026-05-26 estate tech-debt audit chain
for this repo. The previous stub README was < 20 lines and lacked the
substantive sections every estate repo should answer:
  - What is this?
  - How do I use it (quickstart)?
  - What's in here (component layout)?
  - Status / maturity / licence.
  - Companion repositories.

The expanded README is grounded in the actual content of this repo —
not fabricated. Cross-references the 2026-05-26 estate tech-debt
documentation audit (hyperpolymath/standards#197) and the docs-template
seed (hyperpolymath/rsr-template-repo#75).

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath