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/Cargo.toml

@@ -17,3 +17,4 @@ tokio = { version = "1", features = ["rt-multi-thread", "macros", "time", "signa
 sysinfo = "0.30"
 local-ip-address = "0.6"
 zbus = "3"
+hostname = "0.4"