chore: drop ./xport.schema.json export

Nothing in the fleet imports `@socketsecurity/lib/xport.schema.json`
— each repo (socket-btm, socket-sdxgen, socket-tui, socket-packageurl-js,
ultrathink, socket-registry) regenerates its own copy from a TypeBox
spec via scripts/xport-emit-schema.mts. The export is dead surface;
removing it before it ever ships avoids a future major-bump removal.

The xport.schema.json file stays at the repo root (referenced by $id
URL for IDE autocomplete in YAML/JSON files); only the package.json
exports entry is dropped.

Author: jdalton

package.json

@@ -679,8 +679,7 @@
     "./package.json": "./package.json",
     "./tsconfig.dts.json": "./tsconfig.dts.json",
     "./tsconfig.json": "./tsconfig.json",
-    "./tsconfig.test.json": "./tsconfig.test.json",
-    "./xport.schema.json": "./xport.schema.json"
+    "./tsconfig.test.json": "./tsconfig.test.json"
   },
   "files": [
     "dist",