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>

tests/test_integration.py

@@ -95,7 +95,7 @@ class _Harness:
             config["channels"] = channel_config
 
         self.registry = PluginRegistry()
-        self.bot = Bot(config, self.registry)
+        self.bot = Bot("test", config, self.registry)
         self.conn = _MockConnection()
         self.bot.conn = self.conn  # type: ignore[assignment]
         self.bot.state = StateStore(":memory:")