Fixes #242 - Add Express Integration Example to SDK

[davideast]

This PR introduces a new SDK example for Express integrations, as part of the broader effort to add practical Jules TypeScript SDK examples.

It adds a new workspace package at packages/core/examples/express demonstrating:

• Setting up an Express application.
• Creating a REST endpoint (POST /api/jules) that accepts GitHub parameters and a task description.
• Spawning a Jules session asynchronously within the Express route.
• Properly returning session IDs or error states to the client.

It also includes a runnable self-test in index.ts to verify the endpoint functions correctly when executed locally. The root core README.md has been updated to link to this new example.

Fixes #242

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PR created automatically by Jules for task 9293295359877543356 started by @davideast

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