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/idf_component.yml

@@ -2,3 +2,4 @@
 dependencies:
   idf: ">=4.4.1"
   esp_csi_gain_ctrl: ">=0.1.4"
+  espressif/mdns: ">=1.0.0"