Fixes #236: Added GitHub Action AgentSkills Example

[davideast]

Fixes #236

This PR adds a practical example of using the Jules TypeScript SDK inside a scheduled GitHub Action cron job to analyze the repository for Agent Skills that can improve automation.

• Created packages/core/examples/github-action-agentskills directory containing the index.ts script logic, a package.json, tsconfig.json, and action.yml for running as a valid action.
• Added comprehensive documentation on how to use it in README.md.
• Added the example link to the packages/core/README.md.

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PR created automatically by Jules for task 10390679997443409361 started by @davideast

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