fix(infra): simplify alertmanager config mount (direct file mount)

Now that VirtioFS is fixed (colima /Users/mana mount restored),
file bind-mounts work correctly. Replace the cp-in-entrypoint workaround
with a direct file mount to /etc/alertmanager/alertmanager.yml.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker-compose.macmini.yml

@@ -1756,9 +1756,9 @@ services:
     depends_on:
       alert-notifier:
         condition: service_healthy
-    entrypoint: ["sh", "-c", "mkdir -p /tmp/am-config && cp /mnt/alertmanager-config/*.yml /tmp/am-config/ 2>/dev/null; exec /bin/alertmanager --config.file=/tmp/am-config/alertmanager.yml --storage.path=/alertmanager --web.listen-address=:9093"]
+    command: ["--config.file=/etc/alertmanager/alertmanager.yml", "--storage.path=/alertmanager", "--web.listen-address=:9093"]
     volumes:
-      - ./docker/alertmanager:/mnt/alertmanager-config:ro
+      - ./docker/alertmanager/alertmanager.yml:/etc/alertmanager/alertmanager.yml:ro
       - alertmanager_data:/alertmanager
     ports:
       - "9093:9093"