feat: implement CLI program with user prompts and tests

- Add main.py with input prompts, bid validation, screen clearing,
  winner announcement
- Add tests/test_main.py with monkeypatched input tests
- Update docs/plan.md to mark Milestone 3 as complete

tests/test_main.py

@@ -0,0 +1,35 @@
+from secret_auction import constants
+from secret_auction.main import ask_bid, ask_more_bidders, main
+
+
+def feed(monkeypatch, answers):
+    it = iter(answers)
+    monkeypatch.setattr("builtins.input", lambda _prompt="": next(it))
+
+
+def test_ask_bid_reprompts_on_invalid(monkeypatch, capsys):
+    feed(monkeypatch, ["abc", "-5", "nan", "12.5"])
+    assert ask_bid() == 12.5
+    assert capsys.readouterr().out.count(constants.MSG_INVALID_BID) == 3
+
+
+def test_ask_more_bidders(monkeypatch, capsys):
+    feed(monkeypatch, ["maybe", "Y"])
+    assert ask_more_bidders() is True
+    assert constants.MSG_INVALID_ANSWER in capsys.readouterr().out
+    feed(monkeypatch, ["no"])
+    assert ask_more_bidders() is False
+
+
+def test_main_announces_highest_bidder_and_clears_screen(monkeypatch, capsys):
+    feed(monkeypatch, ["Ann", "10", "yes", "Bob", "25.5", "yes", "Cy", "3", "no"])
+    main()
+    out = capsys.readouterr().out
+    assert "The winner is Bob with a bid of $25.50." in out
+    assert out.count("\n" * constants.CLEAR_SCREEN_LINES) == 2
+
+
+def test_main_single_bidder(monkeypatch, capsys):
+    feed(monkeypatch, ["Ann", "7", "n"])
+    main()
+    assert "The winner is Ann with a bid of $7.00." in capsys.readouterr().out