Simplify cleaning transformers and shorten module names

Move cleaning transformers into classifiers/cleaning.py (dropping the
separate cleaning package) and implement them as plain classes rather
than BaseEstimator/TransformerMixin subclasses, since Pipeline only
needs fit/transform via duck typing. Also rename feature_classifier.py
and bag_of_words.py to features.py and bow.py for brevity.

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

classifiers/bow.py

@@ -0,0 +1,46 @@
+from collections import Counter
+
+import numpy as np
+from sklearn.feature_extraction import DictVectorizer
+from sklearn.linear_model import LogisticRegression
+from sklearn.pipeline import Pipeline
+
+from classifiers.cleaning import LowercaseTransformer, PunctuationRemover
+
+
+class FeatureExtractor:
+    def fit(self, X, y=None):
+        return self
+
+    def transform(self, X):
+        return [self.extract_features(msg) for msg in X]
+
+    def extract_features(self, message):
+        return dict(Counter(message.split()))
+
+
+class BagOfWordsClassifier:
+    def fit(self, X, y):
+        self._pipeline = Pipeline([
+            ("lowercase", LowercaseTransformer()),
+            ("punctuation", PunctuationRemover()),
+            ("features", FeatureExtractor()),
+            ("vectorizer", DictVectorizer()),
+            ("classifier", LogisticRegression(max_iter=1000)),
+        ])
+        y_binary = (np.array(y) == "spam").astype(int)
+        self._pipeline.fit(X, y_binary)
+        return self
+
+    def predict(self, X):
+        y_binary = self._pipeline.predict(X)
+        return np.where(y_binary == 1, "spam", "ham")
+
+    def feature_weights(self, top_n=10):
+        vectorizer = self._pipeline.named_steps["vectorizer"]
+        classifier = self._pipeline.named_steps["classifier"]
+        names = vectorizer.get_feature_names_out()
+        weights = classifier.coef_[0]
+        pairs = sorted(zip(names, weights), key=lambda x: x[1])
+        half = top_n // 2
+        return pairs[-half:][::-1] + pairs[:half]