Add 10% OFF discount link for z.ai coding plan

Added discount offer link in multiple locations:
- README.md: In "What is Z.AI API?" section
- install-claude-code.sh: In hero banner, API key prompt, and manual instructions

Discount link: https://z.ai/subscribe?ic=R0K78RJKNW

install-claude-code.sh

@@ -80,6 +80,8 @@ print_banner() {
 EOF
     echo -e "${NC}"
     echo ""
+    echo -e "${MAGENTA}${BOLD}🎁 Grab 10% OFF Token for z.ai coding plan:${NC} ${YELLOW}https://z.ai/subscribe?ic=R0K78RJKNW${NC}"
+    echo ""
     echo -e "${CYAN}Official Documentation:${NC} https://docs.z.ai/devpack/tool/claude"
     echo ""
 }
@@ -147,6 +149,8 @@ get_zai_api_key() {
     echo ""
     echo -e "${YELLOW}Get your API key from:${NC} https://docs.z.ai/devpack/tool/claude"
     echo ""
+    echo -e "${MAGENTA}${BOLD}🎁 Grab 10% OFF Token for z.ai coding plan:${NC} ${YELLOW}https://z.ai/subscribe?ic=R0K78RJKNW${NC}"
+    echo ""
     echo -e "${MAGENTA}Your API key format is: ${BOLD}your-api-key-here${NC}"
     echo ""
 
@@ -252,6 +256,9 @@ show_manual_instructions() {
     echo "  Visit: https://docs.z.ai/devpack/tool/claude"
     echo "  Sign up and get your API key"
     echo ""
+    echo -e "${MAGENTA}${BOLD}🎁 Grab 10% OFF Token for z.ai coding plan:${NC}"
+    echo "  ${YELLOW}https://z.ai/subscribe?ic=R0K78RJKNW${NC}"
+    echo ""
     echo -e "${GREEN}Step 3: Configure Z.AI API${NC}"
     echo "─────────────────────────────────────────────────────────────────"
     echo ""