v20260719-mavericks.1

docs: README.md points to installed README.txt

The repo front page duplicated the setup/usage prose that already ships in
payload/README.txt (installed to /usr/local/share/mavericks-docker/README.txt).
Collapse README.md to a pointer at that single source instead of maintaining two
copies.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>