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/sdkconfig.sample

@@ -437,6 +437,7 @@ CONFIG_PARTITION_TABLE_MD5=y
 CONFIG_CSI_UDP_TARGET_IP="192.168.129.11"
 CONFIG_CSI_UDP_TARGET_PORT=5500
 CONFIG_CSI_CMD_PORT=5501
+CONFIG_CSI_HOSTNAME="your-hostname-here"
 # end of CSI UDP Configuration
 
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