feat: add !uptime command

Track bot start time via monotonic clock, display as compact
duration (e.g. "up 3d 2h 15m 42s").

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

plugins/core.py

@@ -56,6 +56,26 @@ async def cmd_version(bot, message):
     await bot.reply(message, f"derp {__version__}")
 
 
+@command("uptime", help="Show how long the bot has been running")
+async def cmd_uptime(bot, message):
+    """Report bot uptime."""
+    import time
+
+    elapsed = int(time.monotonic() - bot._started)
+    days, rem = divmod(elapsed, 86400)
+    hours, rem = divmod(rem, 3600)
+    minutes, secs = divmod(rem, 60)
+    parts = []
+    if days:
+        parts.append(f"{days}d")
+    if hours:
+        parts.append(f"{hours}h")
+    if minutes:
+        parts.append(f"{minutes}m")
+    parts.append(f"{secs}s")
+    await bot.reply(message, f"up {' '.join(parts)}")
+
+
 @command("load", help="Hot-load a plugin: !load <name>")
 async def cmd_load(bot, message):
     """Load a new plugin from the plugins directory."""