fix(unattended-upgrades): Debian has no reboot-sentinel package, so ship the writer

The first real run failed on all three nodes:

  fatal: [k3s-prod-1]: "No package matching 'update-notifier-common' is available"

update-notifier-common is an UBUNTU package and does not exist in Debian --
`apt-cache policy` returns Candidate: (none) on trixie. The role asserted it
"owns the hook that creates /var/run/reboot-required", which is true on Ubuntu
and false here, so the role could never have worked.

Three further measurements settled the replacement: notify-reboot-required is
absent; NO linux-image-* postinst references it (invoking it is an Ubuntu patch,
so dropping a script at that path would never be called); and no hook under
apt.conf.d or /etc/kernel mentions reboot-required. Debian trixie ships no
mechanism at all.

So the role now provides it:
* /usr/local/sbin/k3s-notify-reboot-required -- touches the sentinel when the
  running kernel is not the newest installed, using `linux-version` (from
  linux-base, a dependency of every linux-image, measured present) because
  kernel version ordering is not string ordering.
* /etc/apt/apt.conf.d/99-k3s-reboot-required -- DPkg::Post-Invoke, `|| true` so
  it can never abort an apt transaction.
/run is tmpfs (measured), so the sentinel self-clears on boot; no cleanup task.

The verify assert was ALSO weaker than it looked: a `find` glob for
*update-notifier* / *unattended-upgrades* under apt.conf.d would have matched
Debian's own unattended-upgrades conffile and reported "writer present" while
none existed. It now stats the two exact paths this role owns and requires the
script to be executable.

Newly verified: the script ran on prod-1 as unprivileged k3s under sh -x --
running=newest=7.1.8+deb13-amd64, took the equal branch, exited 0, wrote
nothing. Correct. sh -n and bash -n clean.

Still unverified: the WRITING half never executes while running == newest, so
the sentinel has not been seen created and kured has not been seen react. The
honest test is the next kernel upgrade.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

decisions/2026-10-17-automated-upgrades.md

@@ -185,6 +185,55 @@ Measured against the live cluster and nodes, read-only, on 2026-10-17.
 - **`renovate.json` parses** as valid JSON after my edits (`json.load`).
 - **kured is not installed** and the string `kured` appears nowhere in the repo — greenfield.
 
+### CORRECTION 2026-10-18 — the first real run FAILED, and the sentinel design was wrong
+
+`ansible-playbook playbooks/unattended-upgrades.yml -e k3s_hosts=k3s_prod` failed on all three
+nodes at the package task:
+
+```
+fatal: [k3s-prod-1]: FAILED! => "No package matching 'update-notifier-common' is available"
+```
+
+**`update-notifier-common` is an UBUNTU package and does not exist in Debian.** Measured on trixie:
+`apt-cache policy update-notifier-common` → `Candidate: (none)`. The original entry asserted this
+package "owns the hook that creates /var/run/reboot-required" — true on Ubuntu, false here, and the
+role could never have worked. This was the exact risk the first version of this entry flagged ("the
+`find` patterns are a guess at which package ships what") landing in a worse place than predicted:
+not a wrong glob, a nonexistent package.
+
+Three further measurements settled what to do instead:
+
+* `/usr/share/update-notifier/notify-reboot-required` is **absent**.
+* **No `linux-image-*` postinst references `notify-reboot-required`** — invoking it is an Ubuntu
+  patch, so merely placing a script at that path would never be called.
+* No hook under `/etc/apt/apt.conf.d` or `/etc/kernel/` mentions `reboot-required`.
+
+So Debian trixie has **no stock mechanism at all**, and the role now ships the writer itself:
+`/usr/local/sbin/k3s-notify-reboot-required` (compares the running kernel against the newest
+installed via `linux-version`, from `linux-base`, which every `linux-image` depends on — measured
+`linux-base 4.12.1` present) plus a `DPkg::Post-Invoke` hook at
+`/etc/apt/apt.conf.d/99-k3s-reboot-required`. `/run` is a tmpfs (measured), so the sentinel
+self-clears on boot and needs no cleanup task.
+
+The verify assert changed shape too: it was a `find` glob over `/etc/kernel/postinst.d` and
+`/etc/apt/apt.conf.d` for `*update-notifier*` / `*unattended-upgrades*`. That glob was **weaker
+than it looked** — it would have matched Debian's own `unattended-upgrades` apt conffile and
+reported "sentinel writer present" while no writer existed. It now stats the two exact paths this
+role owns and requires the script to be executable.
+
+**Newly verified (this correction):** the sentinel script was executed on prod-1 as the
+unprivileged `k3s` user under `sh -x`. It resolved `running=7.1.8+deb13-amd64`,
+`newest=7.1.8+deb13-amd64`, took the equal branch, exited 0 and wrote nothing — correct, since the
+running kernel *is* the newest. `sh -n` and `bash -n` clean.
+
+**Still NOT verified after this correction:** the *writing* half. Because running == newest on all
+three nodes, the `touch` path has never executed, so I have not seen the sentinel actually created,
+nor seen kured react to it. The honest test is the operator's next kernel upgrade. Also unverified:
+whether a `DPkg::Post-Invoke` hook fires in the `unattended-upgrades` code path specifically (it
+runs dpkg, so it should, but that is inference) — and whether `linux-version sort --reverse` orders
+a mixed backports/stable kernel set the way I assume when a 6.12 and a 7.1 are both installed. It
+produced the right answer with the current set; a future set could differ.
+
 ## What I did NOT verify
 
 The honest list. Most of this needs an actual upgrade cycle, which I did not and could not run.