Arcyd: rename cmd 'multi' to 'process-repos'

Rename the 'arcyd multi' command to 'arcyd process-repos' now that it's
the only way to invoke arcyd then it makes more sense.

Test Plan: $ ./precommit.sh
Arcyd: update usage example

Test Plan: didn't test it :/
Arcyd: add trivial 'status-html' command

Pave the way for rich repo-status pages by providing a facility to
transform the content of the touch files into a dashboard page.

Add 'status-html' call to exercise_arcyon.sh

Test Plan: $ ./precommit.sh