fix: load entity secret variables from .env

[arunimshukla]

Problem

The entity-secret setup README tells users to put CIRCLE_API_KEY in .env, but the documented npx tsx index.ts command does not load that file. The script then reports that CIRCLE_API_KEY is missing even when the user followed the setup instructions.

Changes

• Add a start script that runs tsx --env-file=.env index.ts.
• Update the README to use npm start.

Validation

• Parsed the updated package.json successfully.
• Confirmed the start script explicitly loads .env.
• Confirmed the README uses the new command and no longer documents the command that skips .env.
• Confirmed the commit is based directly on the current master head.

The example already requires Node.js 22, so the native --env-file flag is available without adding another dependency.