feat: add !paste command and unit tests for 5 core plugins

Add cmd_paste to flaskpaste plugin (create paste, return URL).
Add test suites for encode, hash, defang, cidr, and dns plugins
(83 new test cases, 1093 total).

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

docs/CHEATSHEET.md

@@ -314,6 +314,7 @@ VT rate limit: 4 req/min. Email check: max 5, admin only.
 
 ```
 !shorten https://long-url.com/x   # Shorten URL
+!paste some text to paste          # Create paste
 ```
 
 Auto-shortens URLs in `!alert` announcements and history when plugin is loaded.