test: add channel filter and JSON log tests

TestChannelFilter: allowed/denied/PM/no-config/core-exempt/ampersand.
TestChannelConfig: TOML loading, defaults. TestJsonFormatter: fields,
exception, unicode, single-line, timestamp format.

tests/test_config.py

@@ -70,6 +70,25 @@ class TestLoad:
         assert result["bot"]["channels"] == ["#test"]
 
 
+class TestChannelConfig:
+    """Test channel-level configuration merging."""
+
+    def test_channel_config_loaded(self, tmp_path: Path):
+        config_file = tmp_path / "test.toml"
+        config_file.write_text(
+            '[channels."#ops"]\nplugins = ["core", "dns"]\n'
+        )
+        result = load(config_file)
+        assert "#ops" in result["channels"]
+        assert result["channels"]["#ops"]["plugins"] == ["core", "dns"]
+
+    def test_default_channels_empty(self):
+        assert DEFAULTS["channels"] == {}
+
+    def test_default_logging_format(self):
+        assert DEFAULTS["logging"]["format"] == "text"
+
+
 class TestResolveConfig:
     """Test config path resolution and fallback."""