Plan project and set up Phase 1 skeleton

Add the framework submodule and the accepted planning documents:
Business Case, Stakeholder Analysis, Project Plan, three milestones,
review records RC-001 to RC-006 and the traceability matrix.

Set up the project skeleton for MIL-001: pyproject.toml (Python
>=3.13, no runtime dependencies, pytest/ruff/mypy as dev extra),
 src/coffee_machine/constants.py, a constants consistency test,
 Doxyfile and a CI workflow that runs ruff, mypy and pytest.

Task: MIL-001#1
Task: MIL-001#2
Task: MIL-001#3
Task: MIL-001#4
Task: MIL-001#5
Closes #1
Closes #2
Closes #3
Closes #4
Closes #5

tests/test_constants.py

@@ -0,0 +1,20 @@
+"""! @file
+@brief Checks that the constants are consistent with each other.
+"""
+
+from coffee_machine import constants
+
+
+def test_every_drink_uses_known_ingredients() -> None:
+    known = set(constants.INITIAL_RESOURCES)
+    for drink in constants.MENU.values():
+        assert set(drink["ingredients"]) <= known  # type: ignore[call-overload]
+
+
+def test_every_resource_has_a_unit() -> None:
+    assert set(constants.INITIAL_RESOURCES) == set(constants.RESOURCE_UNITS)
+
+
+def test_commands_are_not_drink_names() -> None:
+    assert constants.COMMAND_OFF not in constants.MENU
+    assert constants.COMMAND_REPORT not in constants.MENU