test+ci: add regression tests and GitHub Actions workflow

tests/test_filter.py

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+from __future__ import annotations
+
+from twitter_cli.filter import filter_tweets
+
+
+def test_filter_tweets_does_not_mutate_input(tweet_factory) -> None:
+    tweet = tweet_factory("1", score=0.0)
+    output = filter_tweets([tweet], {"mode": "all", "weights": {}})
+
+    assert tweet.score == 0.0
+    assert output[0].score > 0.0
+    assert output[0] is not tweet
+
+
+def test_filter_tweets_applies_language_and_retweet_filters(tweet_factory) -> None:
+    tweets = [
+        tweet_factory("1", lang="en", is_retweet=False),
+        tweet_factory("2", lang="zh", is_retweet=False),
+        tweet_factory("3", lang="en", is_retweet=True),
+    ]
+    output = filter_tweets(
+        tweets,
+        {
+            "mode": "all",
+            "lang": ["en"],
+            "excludeRetweets": True,
+            "weights": {},
+        },
+    )
+
+    assert [tweet.id for tweet in output] == ["1"]