Generate alertmanager.yml so no-webhook is a valid state, not a broken one

Deploying before the webhook exists is deliberate, so it must not depend on Alertmanager tolerating an empty
webhook_url_file — which I could not verify. configure.py now generates the config and omits the Discord
notifier entirely when ALERT_WEBHOOK_URL is unset: the receiver exists, drops notifications, and alerts still
evaluate, group and show at /alerts. Setting the variable and re-applying turns delivery on. The URL stays in
/etc/vanguard/alert-webhook rather than the config, because that file can be 0640 root:65534 for the
container's user while the repo copy cannot.

alertmanager.example.yml is the reference copy and is gitignored in generated form, like the other configs.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

LOGGING.md

@@ -104,8 +104,9 @@ vmalert → Alertmanager → dumpd `/heartbeat`, and the disks. It speaks on cha
 with no webhook configured still logs and exits non-zero so `systemctl is-failed vanguard-deadman.service` is
 a signal. Nothing inside the stack can report its own death; this can.
 
-`ALERT_WEBHOOK_URL` in `.env` → `/etc/vanguard/alert-webhook` (0640 root:65534). Empty means log-only, which
-is a working configuration, not an error.
+`ALERT_WEBHOOK_URL` in `.env` → `/etc/vanguard/alert-webhook` (0640 root:65534). Empty means log-only, which is
+a working configuration, not an error: `configure.py` generates `observability/alertmanager.yml` with the
+Discord notifier omitted, so nothing is half-configured and nothing fails to load.
 
 ## Wiring sketches (the options that were weighed)