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/USAGE.md

@@ -49,9 +49,26 @@ level = "info"                # Logging level: debug, info, warning, error
 | `!ping` | Bot responds with "pong" |
 | `!help` | List all available commands |
 | `!help <cmd>` | Show help for a specific command |
+| `!help <plugin>` | Show plugin description and its commands |
 | `!version` | Show bot version |
 | `!echo <text>` | Echo back text (example plugin) |
 | `!cert <domain> [...]` | Lookup CT logs for up to 5 domains |
+| `!load <plugin>` | Hot-load a plugin from the plugins directory |
+| `!reload <plugin>` | Reload a plugin, picking up file changes |
+| `!unload <plugin>` | Unload a plugin, removing its handlers |
+| `!plugins` | List loaded plugins with handler counts |
+
+### Command Shorthand
+
+Commands can be abbreviated to any unambiguous prefix:
+
+```
+!h           -> !help (unique match)
+!pi          -> !ping (unique match)
+!p           -> error: ambiguous (ping, plugins)
+```
+
+Exact matches always take priority over prefix matches.
 
 ### `!cert` -- Certificate Transparency Lookup
 
@@ -76,6 +93,21 @@ broken.test -- error: timeout
 - crt.sh can be slow; the bot confirms receipt before querying
 - Live cert check runs only when expired CT entries exist
 
+## Plugin Management
+
+Plugins can be loaded, unloaded, and reloaded at runtime without
+restarting the bot.
+
+```
+!load crtsh          # Hot-load a new plugin from plugins/
+!reload crtsh        # Reload a changed plugin
+!unload crtsh        # Remove a plugin and all its handlers
+!plugins             # List loaded plugins with handler counts
+```
+
+The `core` plugin cannot be unloaded (prevents losing `!load`/`!reload`),
+but it can be reloaded.
+
 ## Writing Plugins
 
 Create a `.py` file in the `plugins/` directory: