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

.github/workflows/ci.yml

@@ -0,0 +1,26 @@
+# Runs on GitHub Actions and on Gitea Actions (Gitea also reads .github/workflows).
+name: CI
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.13"
+      - name: Install
+        run: |
+          python -m venv .venv
+          .venv/bin/python -m pip install --upgrade pip
+          .venv/bin/python -m pip install -e ".[dev]"
+      - name: Lint
+        run: .venv/bin/ruff check .
+      - name: Type check
+        run: .venv/bin/mypy
+      - name: Test
+        run: .venv/bin/python -m pytest