feat: remote commands, systemd units, process observability, broker auth split

- Command intake (reboot/shutdown) on infoscreen/{uuid}/commands with ack lifecycle
- MQTT_USER/MQTT_PASSWORD_BROKER split from identity vars; configure_mqtt_security() updated
- infoscreen-simclient.service: Type=notify, WatchdogSec=60, Restart=on-failure
- infoscreen-notify-failure@.service + script: retained MQTT alert when systemd gives up (Gap 3)
- _sd_notify() watchdog keepalive in simclient main loop (Gap 1)
- broker_connection block in health payload: reconnect_count, last_disconnect_at (Gap 2)
- COMMAND_MOCK_REBOOT_IMMEDIATE_COMPLETE canary flag with safety guard
- SERVER_TEAM_ACTIONS.md: server-side integration action items
- Docs: README, CHANGELOG, src/README, copilot-instructions updated
- 43 tests passing

implementation-plans/reboot-command-payload-schemas.md

@@ -0,0 +1,59 @@
+## Reboot Command Payload Schema Snippets
+
+This file provides copy-ready validation snippets for client and integration test helpers.
+
+### Canonical Topics (v1)
+1. Command topic: infoscreen/{client_uuid}/commands
+2. Ack topic: infoscreen/{client_uuid}/commands/ack
+
+### Transitional Compatibility Topics
+1. Command topic alias: infoscreen/{client_uuid}/command
+2. Ack topic alias: infoscreen/{client_uuid}/command/ack
+
+### Canonical Action Values
+1. reboot_host
+2. shutdown_host
+
+### Ack Status Values
+1. accepted
+2. execution_started
+3. completed
+4. failed
+
+### JSON Schema Source
+Use this file for machine validation:
+1. implementation-plans/reboot-command-payload-schemas.json
+
+### Minimal Command Schema Snippet
+```json
+{
+  "type": "object",
+  "additionalProperties": false,
+  "required": ["schema_version", "command_id", "client_uuid", "action", "issued_at", "expires_at", "requested_by", "reason"],
+  "properties": {
+    "schema_version": { "const": "1.0" },
+    "command_id": { "type": "string", "format": "uuid" },
+    "client_uuid": { "type": "string", "format": "uuid" },
+    "action": { "enum": ["reboot_host", "shutdown_host"] },
+    "issued_at": { "type": "string", "format": "date-time" },
+    "expires_at": { "type": "string", "format": "date-time" },
+    "requested_by": { "type": ["integer", "null"] },
+    "reason": { "type": ["string", "null"] }
+  }
+}
+```
+
+### Minimal Ack Schema Snippet
+```json
+{
+  "type": "object",
+  "additionalProperties": false,
+  "required": ["command_id", "status", "error_code", "error_message"],
+  "properties": {
+    "command_id": { "type": "string", "format": "uuid" },
+    "status": { "enum": ["accepted", "execution_started", "completed", "failed"] },
+    "error_code": { "type": ["string", "null"] },
+    "error_message": { "type": ["string", "null"] }
+  }
+}
+```