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.lock

@@ -672,6 +672,17 @@ version = "0.4.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7f24254aa9a54b5c858eaee2f5bccdb46aaf0e486a595ed5fd8f86ba55232a70"
 
+[[package]]
+name = "hostname"
+version = "0.4.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "617aaa3557aef3810a6369d0a99fac8a080891b68bd9f9812a1eeda0c0730cbd"
+dependencies = [
+ "cfg-if",
+ "libc",
+ "windows-link",
+]
+
 [[package]]
 name = "ident_case"
 version = "1.0.1"
@@ -930,6 +941,7 @@ name = "pilot-v2"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "hostname",
  "local-ip-address",
  "rumqttc",
  "serde",