feat(mcp): switch to YAML serialization for tool outputs

- Replaced serde_json with serde_yaml in all handler endpoints.

- This drastically reduces token consumption for large list/graph responses by removing JSON braces and quotes.

server/src/handlers/vision.rs

@@ -758,7 +758,7 @@ impl McpTool for ClipboardHandler {
                 );
 
                 if let Some(img) = item {
-                    Ok(serde_json::to_string_pretty(&img)?)
+                    Ok(serde_yaml::to_string(&img)?)
                 } else {
                     Ok(json!({
                         "status": "no_image_in_clipboard",
@@ -786,7 +786,7 @@ impl McpTool for ClipboardHandler {
 
                 state.record_activity("clipboard", "Retrieved text from clipboard cache", None);
                 if let Some(txt) = item {
-                    Ok(serde_json::to_string_pretty(&txt)?)
+                    Ok(serde_yaml::to_string(&txt)?)
                 } else {
                     Ok(json!({
                         "text": "",
@@ -818,7 +818,7 @@ impl McpTool for ClipboardHandler {
                 };
 
                 state.record_activity("clipboard", "Viewed clipboard history ring buffer", None);
-                Ok(serde_json::to_string_pretty(&history)?)
+                Ok(serde_yaml::to_string(&history)?)
             }
             ClipboardAction::Clear => {
                 tokio::task::spawn_blocking(|| {
@@ -905,7 +905,7 @@ impl McpTool for ClipboardHandler {
                 .map_err(|e| crate::error::AppError::Internal(format!("Task panic: {}", e)))??;
 
                 state.record_activity("clipboard", "Read contents from OS clipboard", None);
-                Ok::<String, crate::error::AppError>(serde_json::to_string_pretty(&Value::Object(
+                Ok::<String, crate::error::AppError>(serde_yaml::to_string(&Value::Object(
                     out,
                 ))?)
             }