feat: Add mDNS, watchdog, human-readable uptime, esp-fleet tool

Firmware:
- mDNS announcement as <hostname>.local (configurable via Kconfig)
- Task watchdog with 30s timeout and auto-reboot on hang
- STATUS now returns human-readable uptime (e.g., 3d2h15m) and hostname

Pi-side tools:
- esp-cmd: mDNS hostname resolution (esp-cmd amber-maple.local STATUS)
- esp-fleet: parallel command to all sensors (esp-fleet status)

Tested on amber-maple — mDNS resolves, watchdog active, fleet tool works.

get-started/csi_recv_router/main/Kconfig.projbuild

@@ -20,4 +20,10 @@ menu "CSI UDP Configuration"
         help
             UDP port for receiving management commands (STATUS, REBOOT, etc.).
 
+    config CSI_HOSTNAME
+        string "Device mDNS hostname"
+        default "esp32-csi"
+        help
+            mDNS hostname for this device. Accessible as <hostname>.local on the network.
+
 endmenu