Fix: send null feature_image to Ghost when field is cleared in front-matter

Previously FeatureImage used omitempty so removing it from the markdown
produced no JSON field — Ghost ignored the update and kept the old image.

Change to *string without omitempty: nil pointer marshals to JSON null,
which Ghost interprets as clear-the-field. Also removes the upsert.go
guard that was zeroing FeatureImage on updates, since that guard existed
only to prevent the omitempty-era empty-string from accidentally nuking
the image.

Author: rodchristiansen

cmd/ghostpost/publish.go

@@ -69,6 +69,12 @@ func publishCmd() *cobra.Command {
 				}
 			}
 
+			// featureImagePtr: nil sends JSON null (clears Ghost field); pointer sends the URL.
+			var featureImagePtr *string
+			if meta.FeatureImage != "" {
+				featureImagePtr = &meta.FeatureImage
+			}
+
 			var html bytes.Buffer
 			if err := goldmark.Convert(md, &html); err != nil {
 				return err
@@ -121,7 +127,7 @@ func publishCmd() *cobra.Command {
 				Slug:           meta.Slug,
 				Status:         defaultStatus(meta.Status),
 				HTML:           html.String(),
-				FeatureImage:   meta.FeatureImage,
+				FeatureImage:   featureImagePtr,
 				Tags:           api.WrapTags(meta.Tags),
 				CustomExcerpt:  meta.CustomExcerpt,
 				PublishedAt:    meta.PublishedAt,

internal/api/post.go

@@ -12,7 +12,7 @@ type Post struct {
 	Slug           string      `json:"slug,omitempty"`
 	Status         string      `json:"status,omitempty"`
 	HTML           string      `json:"html"`
-	FeatureImage   string      `json:"feature_image,omitempty"`
+	FeatureImage   *string     `json:"feature_image"`
 	Tags           []tagRef    `json:"tags,omitempty"`
 	CustomExcerpt  string      `json:"custom_excerpt,omitempty"`
 	PublishedAt    string      `json:"published_at,omitempty"`

internal/api/upsert.go

@@ -30,7 +30,6 @@ func Upsert(c *Client, post Post, id string) (string, error) {
 		post.ID = id
 		post.UpdatedAt = current.UpdatedAt // required lock
 		post.Tags = nil                    // leave unchanged
-		post.FeatureImage = ""             // leave unchanged
 
 		if err := c.Put(ctx, "posts/"+id+"/?source=html", postReq{Posts: []Post{post}}, &res); err != nil {
 			return "", err