Add tests and publish repository metadata for MIL-003

Add 19 pytest tests covering the report, resource check, coin
processing, refunds, change, profit, make_coffee and the main loop.
Add a tests package and put the project root on the pytest path.
Doxygen output is clean. The repository description and topics were set
on the git host.

Closes #13
Closes #14
Closes #15
Closes #16
Closes #17

Task: MIL-003#1
Task: MIL-003#2
Task: MIL-003#3
Task: MIL-003#4
Task: MIL-003#5

tests/test_transactions.py

@@ -0,0 +1,56 @@
+"""!
+@file test_transactions.py
+@brief Tests for coin processing and payment.
+"""
+
+import pytest
+
+from coffee_machine.main import is_transaction_successful, process_coins
+from tests.conftest import ScriptInput
+
+
+def test_process_coins_totals_in_cents(script_input: ScriptInput) -> None:
+    """!
+    @brief Quarters, dimes, nickels and pennies add up to the right total.
+    """
+    script_input(["4", "3", "2", "1"])
+    assert process_coins() == 4 * 25 + 3 * 10 + 2 * 5 + 1
+
+
+def test_process_coins_asks_again_on_invalid_count(
+    script_input: ScriptInput, capsys: pytest.CaptureFixture[str]
+) -> None:
+    """!
+    @brief Text and negative counts are rejected and asked again.
+    """
+    script_input(["abc", "-1", "2", "0", "0", "0"])
+    assert process_coins() == 50
+    assert capsys.readouterr().out.count("Please enter a whole number") == 2
+
+
+def test_transaction_refused_when_too_little(
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    """!
+    @brief Too little money is refunded with the refund message.
+    """
+    assert not is_transaction_successful(100, 150)
+    assert "Sorry that's not enough money. Money refunded." in capsys.readouterr().out
+
+
+def test_transaction_accepted_with_exact_payment(
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    """!
+    @brief Exact payment succeeds and gives no change.
+    """
+    assert is_transaction_successful(150, 150)
+    assert "change" not in capsys.readouterr().out
+
+
+def test_transaction_gives_change(capsys: pytest.CaptureFixture[str]) -> None:
+    """!
+    @brief Overpaying succeeds and returns the difference.
+    """
+    assert is_transaction_successful(175, 150)
+    assert capsys.readouterr().out == "Here is $0.25 in change.\n"