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_plugin.py

@@ -384,7 +384,7 @@ class TestPrefixMatch:
             async def _noop(bot, msg):
                 pass
             registry.register_command(name, _noop, plugin="test")
-        return Bot(config, registry)
+        return Bot("test", config, registry)
 
     def test_exact_match(self):
         bot = self._make_bot(["ping", "pong", "plugins"])
@@ -438,7 +438,7 @@ class TestIsAdmin:
             "bot": {"prefix": "!", "channels": [], "plugins_dir": "plugins",
                     "admins": admins or []},
         }
-        bot = Bot(config, PluginRegistry())
+        bot = Bot("test", config, PluginRegistry())
         if opers:
             bot._opers = opers
         return bot
@@ -565,7 +565,7 @@ def _make_test_bot() -> Bot:
                     "nick": "test", "user": "test", "realname": "test"},
         "bot": {"prefix": "!", "channels": [], "plugins_dir": "plugins"},
     }
-    bot = Bot(config, PluginRegistry())
+    bot = Bot("test", config, PluginRegistry())
     bot.conn = _FakeConnection()  # type: ignore[assignment]
     return bot
 
@@ -637,7 +637,7 @@ class TestChannelFilter:
             "bot": {"prefix": "!", "channels": [], "plugins_dir": "plugins"},
             "channels": channels_cfg or {},
         }
-        return Bot(config, PluginRegistry())
+        return Bot("test", config, PluginRegistry())
 
     def test_core_always_allowed(self):
         bot = self._make_bot({"#locked": {"plugins": ["core"]}})