fix: treat absent onboot as no-autostart (Proxmox default) for alert suppression

parseProxmoxOnBoot returned nil when the onboot key was absent from a
valid config, but Proxmox's default is onboot=0 (do not start on boot).
This caused stopped VMs without an explicit onboot line (like
windows-runner and tails-anon) to still generate powered-off alerts
despite not being configured to autostart.

Now returns false for absent keys in non-empty configs. nil is reserved
for empty/failed config fetches and unrecognised values, preserving the
'unknown' fallback for genuine uncertainty.

internal/monitoring/container_parsing.go

@@ -492,15 +492,17 @@ func extractContainerOSType(config map[string]interface{}) string {
 }
 
 // parseProxmoxOnBoot extracts the onboot (autostart) setting from a Proxmox
-// guest config map. Returns nil when the key is absent or unrecognised so
-// callers can distinguish "explicitly off" from "unknown".
+// guest config map. Returns nil only when the config is empty/unavailable or
+// the value is unrecognised. When the onboot key is absent from a valid config,
+// returns false because Proxmox's default is "do not start on boot".
 func parseProxmoxOnBoot(config map[string]interface{}) *bool {
 	if len(config) == 0 {
 		return nil
 	}
 	raw, ok := config["onboot"]
 	if !ok || raw == nil {
-		return nil
+		v := false
+		return &v
 	}
 	s := strings.TrimSpace(fmt.Sprint(raw))
 	if s == "" {

internal/monitoring/container_parsing_test.go

@@ -1965,7 +1965,7 @@ func TestParseProxmoxOnBoot(t *testing.T) {
 	}{
 		{name: "nil config", config: nil, wantNil: true},
 		{name: "empty config", config: map[string]interface{}{}, wantNil: true},
-		{name: "missing key", config: map[string]interface{}{"ostype": "debian"}, wantNil: true},
+		{name: "missing key", config: map[string]interface{}{"ostype": "debian"}, wantVal: false},
 		{name: "onboot 1 (string)", config: map[string]interface{}{"onboot": "1"}, wantVal: true},
 		{name: "onboot 0 (string)", config: map[string]interface{}{"onboot": "0"}, wantVal: false},
 		{name: "onboot true", config: map[string]interface{}{"onboot": "true"}, wantVal: true},

internal/monitoring/monitor_pve_guest_builders.go

@@ -397,9 +397,9 @@ func (m *Monitor) buildVMFromClusterResource(
 }
 
 // fetchVMOnBoot retrieves the onboot (autostart) setting for a stopped VM by
-// fetching its config. Returns nil when the config is unavailable or the
-// onboot key is absent, so callers can distinguish "explicitly off" from
-// "unknown". Uses a type assertion because PVEClientInterface does not include
+// fetching its config. Returns nil only when the config is unavailable or the
+// value is unrecognised. When onboot is absent from a valid config, returns
+// false because Proxmox's default is "do not start on boot". Uses a type assertion because PVEClientInterface does not include
 // GetVMConfig (it is only on the concrete client, matching the pattern in
 // guest_config.go).
 func (m *Monitor) fetchVMOnBoot(ctx context.Context, client PVEClientInterface, node string, vmid int) *bool {