feat: add multi-server support

Connect to multiple IRC servers concurrently from a single config file.
Plugins are loaded once and shared; per-server state is isolated via
separate SQLite databases and per-bot runtime state (bot._pstate).

- Add build_server_configs() for [servers.*] config layout
- Bot.__init__ gains name parameter, _pstate dict for plugin isolation
- cli.py runs multiple bots via asyncio.gather
- 9 stateful plugins migrated from module-level dicts to _ps(bot) pattern
- Backward compatible: legacy [server] config works unchanged

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

plugins/core.py

@@ -58,7 +58,7 @@ async def cmd_help(bot, message):
 @command("version", help="Show bot version")
 async def cmd_version(bot, message):
     """Report the running version."""
-    await bot.reply(message, f"derp {__version__}")
+    await bot.reply(message, f"derp {__version__} ({bot.name})")
 
 
 @command("uptime", help="Show how long the bot has been running")