docs: point repo URLs at compoundingtech org (org move)
All repos moved to the compoundingtech org (only homebrew-convoy stays
myobie). Switch the GitHub repo URLs in the docs:
- README nix install: github:myobie/pty -> github:compoundingtech/pty
- CHANGELOG pty-relay link -> github.com/compoundingtech/pty-relay
Left unchanged (NOT repo URLs): the npm package scope @myobie/pty (the
package is still published under that name — package.json name is
@myobie/pty) and the com.myobie.pty.* launchd Labels (reverse-DNS
identifiers, backwards-compatible with existing installs).