fleet-manager: host uptime metric + reboot-pending status

The manager lets an admin schedule a reboot (desired.reboot_at) but never showed
whether it had happened. Add host uptime to the health snapshot and derive a
reboot status from it.

- proto: HealthSnapshot.uptime_seconds (15); regenerated pb.go (manager + agent copy).
- agent: probe /proc/uptime -> uptime_seconds (with parser + test).
- manager: Health.UptimeSeconds cached in inventory; snapshotToHealth maps it;
  influx writes + whitelists uptime_seconds as a queryable field.
- UI: fmt.js dur() + rebootStatus() — boot_time = last_seen - uptime (manager clock
  frame, no host/manager sync needed), compared to reboot_at. Monitor host detail
  shows an "uptime" row and a "reboot" row: pending in X / overdue by X / rebooted.

Observability-only — works before the reboot ACTUATOR lands (deferred). Once a host
boots after the scheduled time (BMC/manual now, agent later), the badge flips to
"rebooted" automatically.

files/agent/proto/fleet_agent.pb.go

@@ -274,9 +274,10 @@ type HealthSnapshot struct {
 	Selinux         string                 `protobuf:"bytes,9,opt,name=selinux,proto3" json:"selinux,omitempty"`                                   // enforcing|permissive
 	Chrony          string                 `protobuf:"bytes,10,opt,name=chrony,proto3" json:"chrony,omitempty"`                                    // synced|unsynced
 	KioskActive     bool                   `protobuf:"varint,11,opt,name=kiosk_active,json=kioskActive,proto3" json:"kiosk_active,omitempty"`
-	SessionUptime   string                 `protobuf:"bytes,12,opt,name=session_uptime,json=sessionUptime,proto3" json:"session_uptime,omitempty"`
+	SessionUptime   string                 `protobuf:"bytes,12,opt,name=session_uptime,json=sessionUptime,proto3" json:"session_uptime,omitempty"` // KIOSK session uptime (advisory string)
 	ChromiumCrashes int32                  `protobuf:"varint,13,opt,name=chromium_crashes,json=chromiumCrashes,proto3" json:"chromium_crashes,omitempty"`
 	Display         string                 `protobuf:"bytes,14,opt,name=display,proto3" json:"display,omitempty"`
+	UptimeSeconds   int64                  `protobuf:"varint,15,opt,name=uptime_seconds,json=uptimeSeconds,proto3" json:"uptime_seconds,omitempty"` // HOST uptime (seconds since boot) — boot_time =
 	unknownFields   protoimpl.UnknownFields
 	sizeCache       protoimpl.SizeCache
 }
@@ -409,6 +410,13 @@ func (x *HealthSnapshot) GetDisplay() string {
 	return ""
 }
 
+func (x *HealthSnapshot) GetUptimeSeconds() int64 {
+	if x != nil {
+		return x.UptimeSeconds
+	}
+	return 0
+}
+
 type CommandResult struct {
 	state         protoimpl.MessageState `protogen:"open.v1"`
 	CommandId     string                 `protobuf:"bytes,1,opt,name=command_id,json=commandId,proto3" json:"command_id,omitempty"`
@@ -688,7 +696,7 @@ const file_proto_fleet_agent_proto_rawDesc = "" +
 	"\rrootfs_digest\x18\x02 \x01(\tR\frootfsDigest\x12\x16\n" +
 	"\x06kernel\x18\x03 \x01(\tR\x06kernel\x12\x1b\n" +
 	"\tkiosk_url\x18\x04 \x01(\tR\bkioskUrl\x12#\n" +
-	"\rstarship_name\x18\x05 \x01(\tR\fstarshipName\"\xae\x03\n" +
+	"\rstarship_name\x18\x05 \x01(\tR\fstarshipName\"\xd5\x03\n" +
 	"\x0eHealthSnapshot\x12\x17\n" +
 	"\aat_unix\x18\x01 \x01(\x03R\x06atUnix\x12\x19\n" +
 	"\bload_avg\x18\x02 \x01(\x01R\aloadAvg\x12\x17\n" +
@@ -705,7 +713,8 @@ const file_proto_fleet_agent_proto_rawDesc = "" +
 	"\fkiosk_active\x18\v \x01(\bR\vkioskActive\x12%\n" +
 	"\x0esession_uptime\x18\f \x01(\tR\rsessionUptime\x12)\n" +
 	"\x10chromium_crashes\x18\r \x01(\x05R\x0fchromiumCrashes\x12\x18\n" +
-	"\adisplay\x18\x0e \x01(\tR\adisplay\"V\n" +
+	"\adisplay\x18\x0e \x01(\tR\adisplay\x12%\n" +
+	"\x0euptime_seconds\x18\x0f \x01(\x03R\ruptimeSeconds\"V\n" +
 	"\rCommandResult\x12\x1d\n" +
 	"\n" +
 	"command_id\x18\x01 \x01(\tR\tcommandId\x12\x0e\n" +

files/agent/proto/fleet_agent.proto

@@ -54,9 +54,11 @@ message HealthSnapshot {
   string selinux         = 9;  // enforcing|permissive
   string chrony          = 10; // synced|unsynced
   bool   kiosk_active    = 11;
-  string session_uptime  = 12;
+  string session_uptime  = 12;  // KIOSK session uptime (advisory string)
   int32  chromium_crashes = 13;
   string display         = 14;
+  int64  uptime_seconds  = 15;  // HOST uptime (seconds since boot) — boot_time =
+                                // manager receive-time - this; drives reboot-pending
 }
 
 message CommandResult {