docs: add local environment check for examples

[Aurora137]

Summary

• Add a small local environment check script for example users.
• Document required model configuration variables before running examples.
• Remind contributors not to commit real API keys or local .env files.

Test

• Ran python examples/check_env.py without required variables and confirmed missing variable hints.
• Ran the script with placeholder environment variables and confirmed the success message.

Notes

• This change does not modify core agent logic.
• No real API keys are included.