fix: respect $VISUAL and $EDITOR env vars at runtime

Previously, env vars were only consulted during config generation
and baked into conf.yml. At runtime, the config file value was
always used, making it impossible to override the editor via
environment variables.

Now the precedence is: $VISUAL > $EDITOR > conf.yml > auto-detect.

Closes #589

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/config/new_test.go

@@ -7,6 +7,16 @@ import (
 )
 
 func TestNewTrimsWhitespace(t *testing.T) {
+	// clear env vars so they don't override the config file value
+	oldVisual := os.Getenv("VISUAL")
+	oldEditor := os.Getenv("EDITOR")
+	os.Unsetenv("VISUAL")
+	os.Unsetenv("EDITOR")
+	defer func() {
+		os.Setenv("VISUAL", oldVisual)
+		os.Setenv("EDITOR", oldEditor)
+	}()
+
 	// Create a temporary config file with whitespace in editor and pager
 	tmpDir := t.TempDir()
 	configPath := filepath.Join(tmpDir, "config.yml")