Migrate the Chef repo to Cinc Workstation

Cinc Workstation replaces Chef Workstation for driving knife-zero and
Test Kitchen; nodes run (or are moving to) Cinc Client.

- Pin .ruby-version to cinc-workstation
- Move knife config from .chef/ to .cinc/ (Cinc's USER_CONF_DIR) and
  update .gitignore, README, AGENTS and docs accordingly. On-node config
  now lives in /etc/cinc
- Drop the Gemfile/Gemfile.lock; install knife-zero into Cinc
  Workstation's user gem dir with `cinc gem install knife-zero`
- README: rewrite setup/usage for Cinc Workstation
- scripts/postgresql/switch_primary.sh: drop `bundle exec`

.ruby-version

@@ -1 +1 @@
-chef-workstation
+cinc-workstation