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/src/config/mod.rs

@@ -87,8 +87,12 @@ pub fn load() -> Result<Config> {
         if path.exists() {
             let raw = fs::read_to_string(&path)
                 .with_context(|| format!("failed reading config {}", path.display()))?;
-            let cfg: Config = serde_yaml::from_str(&raw)
+            let mut cfg: Config = serde_yaml::from_str(&raw)
                 .with_context(|| format!("failed parsing config {}", path.display()))?;
+
+            // Substituer $hostname par le vrai hostname
+            expand_variables(&mut cfg)?;
+
             validate(&cfg)?;
             return Ok(cfg);
         }
@@ -116,6 +120,39 @@ pub fn base_device_topic(cfg: &Config) -> String {
     format!("{}/{}", base, cfg.device.name)
 }
 
+// Obtient le hostname du système
+fn get_hostname() -> Result<String> {
+    let hostname = hostname::get()
+        .context("failed to get system hostname")?
+        .to_string_lossy()
+        .to_string();
+    Ok(hostname)
+}
+
+// Remplace les variables comme $hostname dans la config
+fn expand_variables(cfg: &mut Config) -> Result<()> {
+    let hostname = get_hostname()?;
+
+    // Remplacer dans device.name
+    if cfg.device.name == "$hostname" {
+        cfg.device.name = hostname.clone();
+    }
+
+    // Remplacer dans device.identifiers
+    for identifier in &mut cfg.device.identifiers {
+        if identifier == "$hostname" {
+            *identifier = hostname.clone();
+        }
+    }
+
+    // Remplacer dans client_id
+    if cfg.mqtt.client_id == "$hostname" {
+        cfg.mqtt.client_id = hostname.clone();
+    }
+
+    Ok(())
+}
+
 // Verifie les champs minimum pour eviter les erreurs au demarrage.
 fn validate(cfg: &Config) -> Result<()> {
     if cfg.device.name.trim().is_empty() {
@@ -180,4 +217,51 @@ publish:
         assert_eq!(cfg.mqtt.port, 1883);
         assert!(cfg.features.commands.dry_run);
     }
+
+    #[test]
+    fn hostname_substitution() {
+        let raw = r#"
+device:
+  name: "$hostname"
+  identifiers: ["$hostname"]
+mqtt:
+  host: "127.0.0.1"
+  port: 1883
+  username: ""
+  password: ""
+  base_topic: "pilot"
+  discovery_prefix: "homeassistant"
+  client_id: "$hostname"
+  keepalive_s: 60
+  qos: 0
+  retain_states: true
+features:
+  telemetry:
+    enabled: true
+    interval_s: 5
+  commands:
+    enabled: true
+    cooldown_s: 2
+    dry_run: true
+    allowlist: ["shutdown"]
+power_backend:
+  linux: "linux_logind_polkit"
+  windows: "windows_service"
+screen_backend:
+  linux: "gnome_busctl"
+  windows: "winapi_session"
+publish:
+  heartbeat_s: 10
+  availability: true
+"#;
+
+        let mut cfg: Config = serde_yaml::from_str(raw).unwrap();
+        expand_variables(&mut cfg).unwrap();
+
+        let hostname = get_hostname().unwrap();
+        assert_eq!(cfg.device.name, hostname);
+        assert_eq!(cfg.device.identifiers[0], hostname);
+        assert_eq!(cfg.mqtt.client_id, hostname);
+        assert_ne!(cfg.device.name, "$hostname");
+    }
 }