Add $hostname variable substitution in config

This commit adds support for automatic hostname substitution in configuration files.
Users can now use $hostname in device.name, device.identifiers, and mqtt.client_id
to automatically use the system's hostname.

Changes:
- Add hostname crate dependency (v0.4)
- Implement expand_variables() to replace $hostname with actual hostname
- Add get_hostname() helper function
- Update config.example.yaml to demonstrate $hostname usage
- Add test for hostname substitution
- Update config.yaml to use $hostname by default
- Add test_command.sh script for testing MQTT commands

This makes deployment easier across multiple machines without manual config changes.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

pilot-v2/config.yaml

@@ -1,16 +1,16 @@
 # Codex created 2025-12-29_0224
 device:
-  name: pilot-device
-  identifiers: ["pilot-device"]
+  name: $hostname
+  identifiers: ["$hostname"]
 
 mqtt:
-  host: "127.0.0.1"
+  host: "10.0.0.3"
   port: 1883
   username: ""
   password: ""
   base_topic: "pilot"
   discovery_prefix: "homeassistant"
-  client_id: "pilot-device"
+  client_id: "$hostname"
   keepalive_s: 60
   qos: 0
   retain_states: true