security: remove hardcoded credentials, untrack .env

- config/system_config.py: defaults are localhost with empty credentials;
  real hosts/secrets must come from env (.env or docker-compose)
- main.py: help text lists env var names instead of real NATS host/user
- doormile_test.py: reads infra config from env instead of literals
- untrack .env, ignore .env/.env.*, add .env.example with keys only
- pytest.ini: testpaths=tests so doormile_test.py isn't collected

Credentials remain in git history and must be rotated.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012AJLYcbTHCe45fyFnMfEin

.gitignore

@@ -7,3 +7,8 @@ venv/
 
 .idea/
 .vscode/
+
+# secrets
+.env
+.env.*
+!.env.example