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/env.rs

@@ -70,7 +70,7 @@ impl McpTool for EnvironmentHandler {
                     .env_fingerprints
                     .read_with(|fps| fps.get(&ns).cloned());
                 if let Some(fp) = data {
-                    let data = Ok::<String, crate::error::AppError>(serde_json::to_string(&fp)?)?;
+                    let data = Ok::<String, crate::error::AppError>(serde_yaml::to_string(&fp)?)?;
                     Ok(data)
                 } else {
                     Ok("{}".to_string())
@@ -118,7 +118,7 @@ impl McpTool for EnvironmentHandler {
             EnvAction::GetDetails => {
                 let data = state.env.environments.read_with(|envs| {
                     let filtered: Vec<_> = envs.iter().filter(|e| e.namespace == ns).collect();
-                    Ok::<String, crate::error::AppError>(serde_json::to_string(&filtered)?)
+                    Ok::<String, crate::error::AppError>(serde_yaml::to_string(&filtered)?)
                 })?;
                 Ok(data)
             }