fix(core): export package metadata (#1761)

## Summary
- Exposes @fusion/core/package.json through package exports so agent
inspection/self-diagnostics that resolve package metadata do not crash
under Node package exports enforcement.

## Test Plan
- corepack pnpm --filter @fusion/core exec node - <<'NODE'
const { createRequire } = require('node:module');
const req = createRequire(process.cwd() + '/packages/cli/dist/bin.js');
const pkg = req('@fusion/core/package.json');
if (pkg.name !== '@fusion/core') throw new Error('unexpected package
name');
console.log(pkg.name, pkg.version);
NODE
- corepack pnpm --filter @fusion/core build

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<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Chores**
* Added a new package subpath export for `package.json`, improving
compatibility for tools and integrations that need to read package
metadata directly.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/core/package.json

@@ -23,7 +23,8 @@
       "types": "./src/gh-cli.ts",
       "source": "./src/gh-cli.ts",
       "import": "./dist/gh-cli.js"
-    }
+    },
+    "./package.json": "./package.json"
   },
   "publishConfig": {
     "access": "public"