fix(pkg): add default condition to exports for CJS resolvers

The exports map declared only `types` and `import` conditions,
which made CJS resolution fail with ERR_PACKAGE_PATH_NOT_EXPORTED
when a CJS caller (or a tooling path-walker like tsx's
resolveTsPaths) landed on the package. Adding a `default`
condition pointing at the same ESM entry lets Node's CJS
resolver find a match and then fail at load time with a
clearer ESM-interop error instead of a misleading
"no exports main defined".

Functionally this does not make the package dual-mode — it's
still `type: module` and the dist is ESM — but it closes the
CJS resolution gap that was breaking downstream tsx users.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: khaliqgant

package.json

@@ -8,11 +8,13 @@
   "exports": {
     ".": {
       "types": "./dist/index.d.ts",
-      "import": "./dist/index.js"
+      "import": "./dist/index.js",
+      "default": "./dist/index.js"
     },
     "./sdk": {
       "types": "./dist/sdk/index.d.ts",
-      "import": "./dist/sdk/index.js"
+      "import": "./dist/sdk/index.js",
+      "default": "./dist/sdk/index.js"
     }
   },
   "bin": {