fix(cli): export package.json for pgai wrapper resolution

The pgai wrapper uses `require.resolve("postgresai/package.json")` to
locate the main CLI package. When the `exports` field was added during
the Bun migration, it restricted imports to only explicitly exported
paths - but `./package.json` wasn't included.

This caused `npx pgai@dev` to fail with "failed to locate postgresai
package" because Node.js enforces exports restrictions.

Add `./package.json` to the exports map to restore wrapper functionality.

Author: claude

cli/package.json

@@ -18,7 +18,8 @@
   },
   "exports": {
     ".": "./dist/bin/postgres-ai.js",
-    "./cli": "./dist/bin/postgres-ai.js"
+    "./cli": "./dist/bin/postgres-ai.js",
+    "./package.json": "./package.json"
   },
   "type": "module",
   "engines": {