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`

environments/development.json

@@ -1,6 +1,6 @@
 {
   "name": "development",
-  "description": "development environment, for example for Vagrant",
+  "description": "development environment, for example for local Test Kitchen runs",
   "json_class": "Chef::Environment",
   "chef_type": "environment",
   "default_attributes": {