fix: add async/timeout to hooks schema and validate in CI

- hooks.schema.json: add async (boolean) and timeout (number) properties
  to hookItem definition, matching fields used in hooks.json
- validate-hooks.js: validate async and timeout types when present
- hooks.test.js: add SessionEnd to required event types check

schemas/hooks.schema.json

@@ -25,6 +25,15 @@
               }
             }
           ]
+        },
+        "async": {
+          "type": "boolean",
+          "description": "Run hook asynchronously in background without blocking"
+        },
+        "timeout": {
+          "type": "number",
+          "minimum": 0,
+          "description": "Timeout in seconds for async hooks"
         }
       }
     },