Fix 5 pre-release bugs

- Help text showed wrong config path (~/.portainer-cli vs ~/.config/portainer-cli)
- --port without colon (e.g. --port 8080) crashed with nil/tcp key; now errors clearly
- --cmd used naive split(' ') breaking quoted args; switched to Shellwords.split
- --env without '=' (e.g. --env FOO) silently passed empty value; now errors clearly
- SSL/connection errors surfaced as raw exception; now caught with actionable message

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/portainer_cli.rb

@@ -36,7 +36,7 @@ module PortainerCli
         portainer-cli exec <endpoint-id> <container>            Open interactive shell (manual endpoint)
           --shell SHELL                                          Shell to use (default: /bin/sh)
 
-      Config file: ~/.portainer-cli/config.yml
+      Config file: ~/.config/portainer-cli/config.yml
     HELP
 
     def self.run(argv)