zerocracy#277 make factbase filename configurable

The factbase filename was hardcoded as "base.fb". If the server or
baza.rb ever changes the filename (or stores multiple factbases),
entry.sh silently fails with a confusing error from judges.

Now accepts three sources (first wins):
  1. The third argument: entry.sh <id> <home> <fb_file>
  2. FB_FILE environment variable
  3. Default: "base.fb" (backwards compatible)

No breaking changes — existing callers without $3 keep working with
the default filename.

Author: VasilevNStas

entry.sh

@@ -13,7 +13,7 @@ fi
 
 home=$2
 if [ -z "${home}" ]; then
-  echo "The second argument must be the directory where 'base.fb' is located"
+  echo "The second argument must be the directory with the factbase"
   exit 1
 fi
 
@@ -22,6 +22,8 @@ if [ ! -d "${home}" ]; then
   exit 1
 fi
 
+fb_file=${3:-${FB_FILE:-base.fb}}
+
 if ! command -v judges &>/dev/null; then
   echo "'judges' executable not found. Make sure the 'judges' gem is installed."
   exit 1
@@ -30,4 +32,4 @@ fi
 self=$(dirname "$(readlink -f "$0")")
 
 judges update --summary --max-cycles=3 --no-log \
-       --option "id=${id}" --lib "${self}/lib" "${self}/judges" "${home}/base.fb"
+       --option "id=${id}" --lib "${self}/lib" "${self}/judges" "${home}/${fb_file}"