feat: add IRCv3 cap negotiation, channel management, state persistence

Implement CAP LS 302 flow with configurable ircv3_caps list, replacing
the minimal SASL-only registration. Parse IRCv3 message tags (@key=value)
with proper value unescaping. Add channel management plugin (kick, ban,
unban, topic, mode) and bot API methods. Add SQLite key-value StateStore
for plugin state persistence with !state inspection command.

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

tests/test_irc.py

@@ -1,6 +1,6 @@
 """Tests for IRC message parsing and formatting."""
 
-from derp.irc import format_msg, parse
+from derp.irc import _parse_tags, _unescape_tag_value, format_msg, parse
 
 
 class TestParse:
@@ -75,6 +75,36 @@ class TestParse:
         assert msg.target == "#channel"
         assert msg.text == "leaving"
 
+    def test_no_tags_default(self):
+        msg = parse(":nick!u@h PRIVMSG #ch :hello")
+        assert msg.tags == {}
+
+    def test_tags_parsed(self):
+        msg = parse("@time=2026-02-15T12:00:00Z;account=alice "
+                     ":nick!user@host PRIVMSG #chan :hello")
+        assert msg.tags == {"time": "2026-02-15T12:00:00Z", "account": "alice"}
+        assert msg.nick == "nick"
+        assert msg.command == "PRIVMSG"
+        assert msg.text == "hello"
+
+    def test_tags_value_unescaping(self):
+        msg = parse(r"@key=hello\sworld\:end :nick!u@h PRIVMSG #ch :test")
+        assert msg.tags["key"] == "hello world;end"
+
+    def test_tags_no_value(self):
+        msg = parse("@draft/feature :nick!u@h PRIVMSG #ch :test")
+        assert msg.tags == {"draft/feature": ""}
+
+    def test_tags_backslash_escapes(self):
+        assert _unescape_tag_value(r"a\\b\r\n") == "a\\b\r\n"
+
+    def test_tags_mixed_keys(self):
+        tags = _parse_tags("a=1;b;c=three")
+        assert tags == {"a": "1", "b": "", "c": "three"}
+
+    def test_tags_empty_string(self):
+        assert _parse_tags("") == {}
+
 
 class TestFormat:
     """IRC message formatting tests."""