Reject empty min_release_age, self-age the e2e fixtures, run self-tests in CI

An empty min_release_age string now maps to unparseable rather than absent,
so "min_release_age": "" fails validation instead of silently running
with a zero-second quarantine. The end-to-end fixtures extend the
checked-in pkgver (.90/.91) so the test keeps working at any future mise
version. A Tests workflow runs bin/sync-upstream self-test and
bin/omarchy-pkgs self-test on every PR in the Arch container, making the
proof machine-checked instead of author-supplied. The README package
metadata field list documents upstream and min_release_age.

.github/workflows/test.yml

@@ -0,0 +1,31 @@
+name: Tests
+
+on:
+  pull_request:
+  push:
+    branches: [master]
+  workflow_dispatch:
+
+jobs:
+  self-tests:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+        with:
+          persist-credentials: false
+
+      # An Arch container for vercmp: version ordering has to be decided by
+      # the same comparator pacman uses on users' machines.
+      - name: Run self-tests
+        run: |
+          docker run --rm \
+            -v "$PWD:/workspace:ro" \
+            -w /workspace \
+            archlinux:base-devel bash -lc '
+              set -euo pipefail
+              pacman -Syu --noconfirm jq
+              ./bin/sync-upstream self-test
+              ./bin/omarchy-pkgs self-test
+            '