feat: add hot-reload, shorthand commands, and plugin help

- Plugin registry: add unload_plugin(), reload_plugin(), path tracking
- Bot: add load_plugin(), reload_plugin(), unload_plugin() public API
- Core plugin: add !load, !reload, !unload, !plugins commands
- Command dispatch: support unambiguous prefix matching (!h -> !help)
- Help: support !help <plugin> to show plugin description and commands
- Tests: 17 new tests covering hot-reload, prefix matching

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

docs/CHEATSHEET.md

@@ -18,9 +18,15 @@ derp -v               # Verbose/debug mode
 !ping              # Pong
 !help              # List commands
 !help <cmd>        # Command help
+!help <plugin>     # Plugin description + commands
 !version           # Bot version
 !echo <text>       # Echo text back
 !cert <domain>     # CT log lookup (max 5 domains)
+!load <plugin>     # Hot-load a plugin
+!reload <plugin>   # Reload a changed plugin
+!unload <plugin>   # Remove a plugin
+!plugins           # List loaded plugins
+!h                 # Shorthand (any unambiguous prefix works)
 ```
 
 ## Plugin Template