Fixes #240: Add Database Triggers Example to SDK

[davideast]

Fixes #240

This PR adds a new example for triggering a Jules session based on a database event using PostgreSQL's LISTEN/NOTIFY.

Changes made

• Created packages/core/examples/database-triggers/package.json with appropriate workspace dependencies and PostgreSQL driver (pg).
• Created packages/core/examples/database-triggers/index.ts to implement a listening script that invokes jules.session() upon receiving a notification payload.
• Added packages/core/examples/database-triggers/README.md containing instructions on how to set up the example and emit a notification using psql.
• Updated packages/core/README.md to link to the new Database Triggers example.

I've ensured to use bun install for dependency setup and ran bun run build and npm run test --workspace=@google/jules-sdk to verify everything works and no tests are broken.

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PR created automatically by Jules for task 2231257463971507578 started by @davideast

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