Break enterprise policy out into openchamber-policy.json

- New --enterprise <policy.json> flag installs a policy file from the
  cloned repo (openchamber-policy.json) instead of the built-in defaults
- --relay-url overrides the source file's relayUrl key; matching relay
  installs the file content verbatim
- Invalid or missing policy source fails closed with a clear error
- Arg parsing extracted into a testable parse_args function

openchamber-policy.json

@@ -0,0 +1,5 @@
+{
+  "enterpriseMode": true,
+  "organization": "Zuccaro",
+  "relayUrl": "wss://code.zuccaro.me/ws"
+}