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>

config/derp.toml.example

@@ -8,6 +8,7 @@ realname = "derp IRC bot"
 password = ""
 # sasl_user = "account"    # SASL PLAIN username (optional)
 # sasl_pass = "secret"     # SASL PLAIN password (optional)
+# ircv3_caps = ["multi-prefix", "away-notify", "server-time"]  # IRCv3 capabilities
 
 [bot]
 prefix = "!"