fix: address coderabbitai review feedback on PR #127

- Add cobra.NoArgs to env list command to reject accidental extra args
- Add package doc comments to health, db, metrics, and cli packages

https://claude.ai/code/session_01Kt2cz2tqC28r9b6bM1c7LE

Author: claude

packages/engine/internal/api/health/health.go

@@ -1,3 +1,4 @@
+// Package health provides HTTP health-check endpoints for the Mantle API server.
 package health
 
 import (

packages/engine/internal/cli/credentials.go

@@ -1,3 +1,4 @@
+// Package cli implements the Mantle command-line interface.
 package cli
 
 import (

packages/engine/internal/cli/env.go

@@ -76,6 +76,7 @@ func newEnvListCommand() *cobra.Command {
 	return &cobra.Command{
 		Use:   "list",
 		Short: "List all environments",
+		Args:  cobra.NoArgs,
 		RunE: func(cmd *cobra.Command, args []string) error {
 			store, cleanup, err := newEnvStore(cmd)
 			if err != nil {

packages/engine/internal/db/db.go

@@ -1,3 +1,4 @@
+// Package db provides Postgres connection management and query helpers.
 package db
 
 import (

packages/engine/internal/metrics/metrics.go

@@ -1,3 +1,4 @@
+// Package metrics defines Prometheus metrics for the Mantle server.
 package metrics
 
 import (