fix: use ${GITEA_TOKEN:-} to handle unset token and initialize env during init

With set -u, expanding $GITEA_TOKEN fails if not set.
Changed to ${GITEA_TOKEN:-} to provide empty default.

Also initializes $ENV_DIR/default.env during kugetsu init
so users are aware of the env file structure.

skills/kugetsu/scripts/kugetsu-session.sh

@@ -58,6 +58,25 @@ EOF
         echo "Created config file: $KUGETSU_DIR/config"
     fi
 
+    mkdir -p "$ENV_DIR"
+    if [ ! -f "$ENV_DIR/default.env" ]; then
+        cat > "$ENV_DIR/default.env" << 'EOF'
+# Environment variables for agents
+# Copy this file to <agent-type>.env (e.g., pm-agent.env, dev.env)
+# and set your tokens and configuration
+
+# Required: Gitea token for API access
+# GITEA_TOKEN=your_gitea_token_here
+
+# Optional: GitHub token (if using GitHub)
+# GITHUB_TOKEN=your_github_token_here
+
+# Optional: GitLab token (if using GitLab)
+# GITLAB_TOKEN=your_gitlab_token_here
+EOF
+        echo "Created env template: $ENV_DIR/default.env"
+    fi
+
     local existing_base=$(get_base_session_id)
     local existing_pm=$(get_pm_agent_session_id)