feat(config): add env map for workflow expression variables (#74)

Add Env map[string]string field to Config struct, parsed from the env:
section in mantle.yaml. Keys are normalized to uppercase to match the
MANTLE_ENV_* convention used by the CEL evaluator.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: michaelmcnees

packages/engine/internal/config/config.go

@@ -8,6 +8,7 @@ import (
 	"log"
 	"net/url"
 	"os"
+	"strings"
 	"time"
 
 	"github.com/dvflw/mantle/internal/budget"
@@ -30,6 +31,7 @@ type Config struct {
 	GCP        GCPConfig        `mapstructure:"gcp"`
 	Azure      AzureConfig      `mapstructure:"azure"`
 	Tmp        TmpConfig        `mapstructure:"tmp"`
+	Env        map[string]string `mapstructure:"env"`
 }
 
 // RetentionConfig holds data retention settings.
@@ -274,6 +276,16 @@ func Load(cmd *cobra.Command) (*Config, error) {
 		return nil, err
 	}
 
+	// Viper lowercases all map keys. Normalize env map keys to uppercase
+	// so they match MANTLE_ENV_* convention and CEL expressions like env.APP_NAME.
+	if len(cfg.Env) > 0 {
+		normalized := make(map[string]string, len(cfg.Env))
+		for k, v := range cfg.Env {
+			normalized[strings.ToUpper(k)] = v
+		}
+		cfg.Env = normalized
+	}
+
 	// Validate budget reset_day range.
 	if cfg.Engine.Budget.ResetDay < 1 || cfg.Engine.Budget.ResetDay > 28 {
 		if cfg.Engine.Budget.ResetMode == budget.ResetModeRolling {

packages/engine/internal/config/config_test.go

@@ -309,3 +309,43 @@ tmp:
 	// path from file should still be present since env var didn't override it
 	assert.Equal(t, "/tmp/file-path", cfg.Tmp.Path)
 }
+
+func TestLoad_EnvMap(t *testing.T) {
+	dir := t.TempDir()
+	configFile := filepath.Join(dir, "mantle.yaml")
+	err := os.WriteFile(configFile, []byte(`
+env:
+  APP_NAME: "my-app"
+  REGION: "us-east-1"
+  DEBUG: "true"
+`), 0644)
+	require.NoError(t, err)
+
+	cmd := newTestCommand()
+	_ = cmd.Flags().Set("config", configFile)
+
+	cfg, err := Load(cmd)
+	require.NoError(t, err)
+
+	assert.Equal(t, "my-app", cfg.Env["APP_NAME"])
+	assert.Equal(t, "us-east-1", cfg.Env["REGION"])
+	assert.Equal(t, "true", cfg.Env["DEBUG"])
+}
+
+func TestLoad_EnvMapEmpty(t *testing.T) {
+	dir := t.TempDir()
+	configFile := filepath.Join(dir, "mantle.yaml")
+	err := os.WriteFile(configFile, []byte(`
+log:
+  level: "debug"
+`), 0644)
+	require.NoError(t, err)
+
+	cmd := newTestCommand()
+	_ = cmd.Flags().Set("config", configFile)
+
+	cfg, err := Load(cmd)
+	require.NoError(t, err)
+
+	assert.Empty(t, cfg.Env)
+}