Per mara on hyperhive#1898: the top-level README should not go into
details about impl details or specific subsystems. Trims the
Multi-account Matrix support and GitHub account sections down to a
one-line pointer each.
The matrixAccounts option detail that was only ever documented in the
README (checked: docs/matrix.md is entirely about the host hive-matrix
container, not this per-agent option) moves to docs/tools/matrix.md
instead, next to the account parameter every matrix tool already
takes - the natural home, not a link to a page that never covered it.
c6af5a2abf