fix(#284): remove dead code across API, models, WS client, and docs (#299)

Remove six dead-code findings from the automated review:
- CODE-01: updateProfileDescription endpoint + UpdateProfileDescriptionRequest DTO (uncalled)
- CODE-02: getConfig() endpoint (app uses getRawConfig())
- CODE-03: TogglePluginRequest DTO (unreferenced)
- CODE-05: mark isConnected with @VisibleForTesting (test-only usage)
- CODE-06: fix KDoc createdAt -> timestamp to match actual field name

Co-authored-by: M57 <hy4ri@users.noreply.github.com>

app/src/main/java/com/m57/hermescontrol/data/model/PluginInfo.kt

@@ -28,10 +28,6 @@ data class PluginsHubResponse(
     val plugins: List<PluginInfo>,
 )
 
-data class TogglePluginRequest(
-    val enabled: Boolean,
-)
-
 data class AgentPluginInstallBody(
     val identifier: String,
     val force: Boolean = false,

app/src/main/java/com/m57/hermescontrol/data/model/Profile.kt

@@ -37,7 +37,3 @@ data class UpdateProfileModelRequest(
     val provider: String,
     val model: String,
 )
-
-data class UpdateProfileDescriptionRequest(
-    val description: String,
-)