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>

plugins/core.py

@@ -1,4 +1,6 @@
-"""Core plugin: ping, help, version."""
+"""Core plugin: ping, help, version, plugin management."""
+
+from collections import Counter
 
 from derp import __version__
 from derp.plugin import command
@@ -10,22 +12,37 @@ async def cmd_ping(bot, message):
     await bot.reply(message, "pong")
 
 
-@command("help", help="List commands or show command help")
+@command("help", help="List commands or show command/plugin help")
 async def cmd_help(bot, message):
-    """Show available commands or help for a specific command.
+    """Show available commands, or help for a specific command or plugin.
 
-    Usage: !help [command]
+    Usage: !help [command|plugin]
     """
     parts = message.text.split(None, 2)
     if len(parts) > 1:
-        # Help for a specific command
-        cmd_name = parts[1].lower().lstrip(bot.prefix)
-        handler = bot.registry.commands.get(cmd_name)
+        name = parts[1].lower().lstrip(bot.prefix)
+
+        # Check command first
+        handler = bot.registry.commands.get(name)
         if handler:
             help_text = handler.help or "No help available."
-            await bot.reply(message, f"{bot.prefix}{cmd_name} -- {help_text}")
-        else:
-            await bot.reply(message, f"Unknown command: {cmd_name}")
+            await bot.reply(message, f"{bot.prefix}{name} -- {help_text}")
+            return
+
+        # Check plugin
+        module = bot.registry._modules.get(name)
+        if module:
+            desc = (getattr(module, "__doc__", "") or "").split("\n")[0].strip()
+            cmds = sorted(
+                k for k, v in bot.registry.commands.items() if v.plugin == name
+            )
+            lines = [f"{name} -- {desc}" if desc else name]
+            if cmds:
+                lines.append(f"Commands: {', '.join(bot.prefix + c for c in cmds)}")
+            await bot.reply(message, " | ".join(lines))
+            return
+
+        await bot.reply(message, f"Unknown command or plugin: {name}")
         return
 
     # List all commands
@@ -37,3 +54,61 @@ async def cmd_help(bot, message):
 async def cmd_version(bot, message):
     """Report the running version."""
     await bot.reply(message, f"derp {__version__}")
+
+
+@command("load", help="Hot-load a plugin: !load <name>")
+async def cmd_load(bot, message):
+    """Load a new plugin from the plugins directory."""
+    parts = message.text.split(None, 2)
+    if len(parts) < 2:
+        await bot.reply(message, "Usage: !load <plugin>")
+        return
+    name = parts[1].lower()
+    ok, reason = bot.load_plugin(name)
+    if ok:
+        await bot.reply(message, f"Loaded plugin: {name} ({reason})")
+    else:
+        await bot.reply(message, f"Failed to load plugin: {reason}")
+
+
+@command("reload", help="Reload a plugin: !reload <name>")
+async def cmd_reload(bot, message):
+    """Unload and reload a plugin, picking up file changes."""
+    parts = message.text.split(None, 2)
+    if len(parts) < 2:
+        await bot.reply(message, "Usage: !reload <plugin>")
+        return
+    name = parts[1].lower()
+    ok, reason = bot.reload_plugin(name)
+    if ok:
+        await bot.reply(message, f"Reloaded plugin: {name}")
+    else:
+        await bot.reply(message, f"Failed to reload plugin: {reason}")
+
+
+@command("unload", help="Unload a plugin: !unload <name>")
+async def cmd_unload(bot, message):
+    """Unload a plugin, removing all its handlers."""
+    parts = message.text.split(None, 2)
+    if len(parts) < 2:
+        await bot.reply(message, "Usage: !unload <plugin>")
+        return
+    name = parts[1].lower()
+    ok, reason = bot.unload_plugin(name)
+    if ok:
+        await bot.reply(message, f"Unloaded plugin: {name}")
+    else:
+        await bot.reply(message, f"Failed to unload plugin: {reason}")
+
+
+@command("plugins", help="List loaded plugins")
+async def cmd_plugins(bot, message):
+    """List all loaded plugins with handler counts."""
+    counts: Counter[str] = Counter()
+    for handler in bot.registry.commands.values():
+        counts[handler.plugin] += 1
+    for handlers in bot.registry.events.values():
+        for handler in handlers:
+            counts[handler.plugin] += 1
+    parts = [f"{name} ({counts[name]})" for name in sorted(counts)]
+    await bot.reply(message, f"Plugins: {', '.join(parts)}")