Add ci-python-app reusable workflow

Reusable CI for plain Python apps / locally-run tooling that don't deploy via
SAM or CDK. Beyond ruff lint/format + the conventions audit, it adds a
collect-only import check for a root suite whose live run needs secrets, and an
isolated full pytest run for a self-contained subproject dir (whose tests/
package would collide with the root tests/ under one rootdir).

Emits the org-required `ci / ci` via an aggregator job keyed `ci` that gates on
every other job. actionlint-clean.

.github/workflows/ci-python-app.yaml

@@ -0,0 +1,150 @@
+name: CI — Python (app)
+
+# Reusable CI for plain Python apps / locally-run tooling that do NOT deploy via
+# SAM or CDK (use ci-python-sam.yaml / ci-typescript-cdk.yaml for those). Beyond
+# lint + format it adds two things such repos commonly need:
+#   * a collect-only import check for a root suite whose live run needs secrets
+#     (verifies every test module imports cleanly without running them), and
+#   * an isolated full pytest run for a self-contained subproject dir whose tests
+#     package collides with the root tests/ package (e.g. a `tests/` under a
+#     subdir) and so must run in its own working directory.
+#
+# Naming is load-bearing (see this repo's ci.yaml): the org ruleset matches the
+# required `ci / ci` check against the JOB check-run name. A caller job keyed `ci`
+# invoking this workflow reports each job here as `ci / <job>`, so the aggregator
+# job below is keyed `ci` to emit exactly `ci / ci`. The aggregator gates on every
+# other job, so the single required check fails if any sub-job fails.
+
+on:
+  workflow_call:
+    inputs:
+      python-version:
+        description: "Python version to use"
+        type: string
+        default: "3.12"
+      source-dirs:
+        description: "Space-separated directories for ruff (default: repo root)"
+        type: string
+        default: "."
+      requirements:
+        description: "Requirements file used for the pip cache key + install"
+        type: string
+        default: "requirements.txt"
+      collect-only:
+        description: "Run 'pytest --collect-only' at the repo root (imports resolve without secrets)"
+        type: boolean
+        default: true
+      subproject-dir:
+        description: "Optional self-contained subproject dir whose pytest suite runs in full"
+        type: string
+        default: ""
+      run-conventions-check:
+        description: "Run the lightweight conventions audit (README + .gitignore covers .env)"
+        type: boolean
+        default: true
+
+permissions:
+  contents: read
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    steps:
+      - uses: actions/checkout@v6
+
+      - uses: actions/setup-python@v6
+        with:
+          python-version: ${{ inputs.python-version }}
+
+      - name: Install ruff
+        run: pip install ruff
+
+      - name: Ruff check
+        run: ruff check ${{ inputs.source-dirs }}
+
+      - name: Ruff format check
+        run: ruff format --check ${{ inputs.source-dirs }}
+
+      - name: Conventions check
+        if: ${{ inputs.run-conventions-check }}
+        run: |
+          errors=0
+          fail() { echo "::error::$1"; errors=$((errors + 1)); }
+
+          # README must exist
+          if [[ ! -f README.md ]]; then
+            fail "Missing README.md"
+          fi
+
+          # .gitignore must cover .env
+          if [[ -f .gitignore ]]; then
+            if ! grep -qE '^\.env$|^\.env\b' .gitignore; then
+              fail ".gitignore does not include .env"
+            fi
+          else
+            fail "Missing .gitignore"
+          fi
+
+          if [[ $errors -gt 0 ]]; then
+            echo "Conventions check failed with $errors error(s)."
+            exit 1
+          fi
+          echo "Conventions check passed."
+
+  test-collect:
+    if: ${{ inputs.collect-only }}
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    steps:
+      - uses: actions/checkout@v6
+
+      - uses: actions/setup-python@v6
+        with:
+          python-version: ${{ inputs.python-version }}
+          cache: pip
+          cache-dependency-path: ${{ inputs.requirements }}
+
+      - name: Install dependencies
+        run: |
+          pip install -r "${{ inputs.requirements }}"
+          pip install pytest python-dotenv
+
+      - name: Pytest collect-only
+        run: pytest --collect-only -q
+
+  subproject-tests:
+    if: ${{ inputs.subproject-dir != '' }}
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    steps:
+      - uses: actions/checkout@v6
+
+      - uses: actions/setup-python@v6
+        with:
+          python-version: ${{ inputs.python-version }}
+          cache: pip
+          cache-dependency-path: ${{ inputs.requirements }}
+
+      - name: Install dependencies
+        run: |
+          pip install -r "${{ inputs.requirements }}"
+          pip install pytest
+
+      - name: Run subproject suite
+        working-directory: ${{ inputs.subproject-dir }}
+        run: python -m pytest -q
+
+  ci:
+    # Aggregator — keyed `ci` so a caller job keyed `ci` reports `ci / ci`.
+    needs: [lint, test-collect, subproject-tests]
+    if: always()
+    runs-on: ubuntu-latest
+    steps:
+      - name: Require all jobs to have succeeded
+        run: |
+          if [ "${{ contains(needs.*.result, 'failure') || contains(needs.*.result, 'cancelled') }}" = "true" ]; then
+            echo "A required CI job failed or was cancelled."
+            exit 1
+          fi
+          echo "All CI jobs passed."