feat: channel key support for +k channels

Add channel_keys dict to NetworkConfig for storing per-channel keys.
Keys are used in KICK rejoin, passed via AUTOJOIN +#channel key syntax,
supported in ADDNETWORK channel_keys= parameter, and propagated through
REHASH. Extract rehash() as reusable async function for SIGHUP reuse.

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

tests/test_network.py

@@ -479,6 +479,25 @@ class TestHandleKick:
         # Should rejoin (rejoin_delay=0)
         writer.write.assert_called_with(b"JOIN #test\r\n")
 
+    @pytest.mark.asyncio
+    async def test_kick_rejoin_with_key(self) -> None:
+        cfg = _cfg(channels=["#secret"])
+        cfg.channel_keys = {"#secret": "hunter2"}
+        net = _net(cfg=cfg)
+        net.state = State.READY
+        net._running = True
+        net.nick = "me"
+        net.channels = {"#secret"}
+        writer = MagicMock()
+        writer.is_closing.return_value = False
+        writer.drain = AsyncMock()
+        net._writer = writer
+
+        await net._handle(_msg(":op!user@host KICK #secret me :reason"))
+        assert "#secret" not in net.channels
+        # Should rejoin with key (rejoin_delay=0)
+        writer.write.assert_called_with(b"JOIN #secret hunter2\r\n")
+
     @pytest.mark.asyncio
     async def test_kick_other_user_ignored(self) -> None:
         net = _net()