fix(files): route UpsertFile to POST when sha is empty so new files can be created

internal/gitea/files.go

@@ -92,13 +92,24 @@ type FileWriteResult struct {
 	} `json:"commit"`
 }
 
+// UpsertFile creates a file when args.Sha is empty (POST) or updates an existing
+// file when args.Sha is set (PUT). Gitea routes both operations by HTTP method on
+// the same /contents/{path} URL, and rejects PUT without a sha.
 func (c *Client) UpsertFile(ctx context.Context, owner, repo, path string, args UpsertFileArgs) (*FileWriteResult, error) {
 	p := fmt.Sprintf("/api/v1/repos/%s/%s/contents/%s", owner, repo, path)
 	payload, err := json.Marshal(args)
 	if err != nil {
 		return nil, err
 	}
-	body, status, err := c.PutJSON(ctx, p, payload)
+	var (
+		body   []byte
+		status int
+	)
+	if args.Sha == "" {
+		body, status, err = c.PostJSON(ctx, p, payload)
+	} else {
+		body, status, err = c.PutJSON(ctx, p, payload)
+	}
 	if err != nil {
 		return nil, err
 	}