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>

src/derp/bot.py

@@ -4,6 +4,7 @@ from __future__ import annotations
 
 import asyncio
 import logging
+import time
 from pathlib import Path
 
 from derp.irc import IRCConnection, Message, format_msg, parse
@@ -30,6 +31,7 @@ class Bot:
         self.nick: str = config["server"]["nick"]
         self.prefix: str = config["bot"]["prefix"]
         self._running = False
+        self._started: float = time.monotonic()
 
     async def start(self) -> None:
         """Connect, register, join channels, and enter the main loop."""