fix: unblock the Ubuntu 26.04 installer (qemu-kvm, sudo-rs) (#665)

setup.sh aborts twice on resolute. qemu-kvm is a virtual package with two
providers there, so apt refuses to resolve it; the token was redundant anyway
since $QEMU_PACKAGES already installs qemu-system-x86/arm and spinifex execs
the qemu-system-* binaries directly.

sudo-rs, Ubuntu's default sudo since 25.10, then rejects the sudoers file: it
implements no pam_session Defaults setting, and forbids a wildcard inside a
command argument. pam_session is now emitted only for classic sudo, and the
sysctl grant names a fixed-verb helper instead of a wildcard.

The helper is narrower than the rule it replaces: it accepts one interface, a
key restricted to rp_filter or accept_local, and a value restricted to 0 or 1,
where the old grant permitted any net.ipv4.conf key at any value. Granting
sysctl with an unconstrained argument was rejected as root-equivalent.

Adds scripts/lint-apt-packages.sh and an Apt Packages workflow so the runtime
package list is resolved against each supported suite's real indices in CI.

.github/workflows/apt-packages.yml

@@ -0,0 +1,48 @@
+name: Apt Packages
+
+# Path-filtered like image-scripts.yml: this resolves setup.sh's runtime package
+# list against each supported suite's real indices, so a renamed, dropped, or
+# ambiguously virtual package fails here rather than on a user's machine.
+on:
+  push:
+    branches:
+      - main
+      - dev
+    paths:
+      - scripts/setup.sh
+      - scripts/lint-apt-packages.sh
+      - .github/workflows/apt-packages.yml
+  pull_request:
+    branches:
+      - main
+      - dev
+    paths:
+      - scripts/setup.sh
+      - scripts/lint-apt-packages.sh
+      - .github/workflows/apt-packages.yml
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  apt_packages:
+    name: Resolve apt list (${{ matrix.image }})
+    runs-on: ubuntu-26.04
+
+    strategy:
+      fail-fast: false
+      matrix:
+        # The suites the nightly E2E matrix actually installs on. Ubuntu 22.04 is
+        # accepted by detect_os but has never been exercised and does not resolve.
+        image:
+          - ubuntu:24.04
+          - ubuntu:26.04
+          - debian:13
+
+    steps:
+      - name: Checkout Code
+        uses: actions/checkout@v7
+
+      - name: Resolve runtime package list
+        run: ./scripts/lint-apt-packages.sh ${{ matrix.image }}