fix(kugetsu): use --format json for session existence check

Use opencode session list --format json with grep instead of table
output to make check_opencode_session_exists() more reliable.

This fixes issue #38 where forked sessions may not appear in the
table format output, causing false 'session expired' reports.

skills/kugetsu/scripts/kugetsu

@@ -303,7 +303,7 @@ validate_issue_ref() {
 
 check_opencode_session_exists() {
     local session_id="$1"
-    opencode session list 2>/dev/null | grep -q "^$session_id"
+    opencode session list --format json 2>/dev/null | grep -q "\"$session_id\""
 }
 
 kugetsu_get_pm_context() {