Add documentation for secret configuration and integration test execution

[Copilot]

Integration tests require API keys but lacked documentation on environment setup and secret management in Copilot environments.

Changes

New: docs/TESTING_WITH_SECRETS.md

• Copilot secret injection mechanism and configuration
• Local development setup with .env file
• Test execution commands with expected durations
• Troubleshooting common errors (missing API key, network issues)
• Environment variables reference

Updated: README.md

• Quick start: install → configure → test
• Test suite overview (unit vs integration)
• Links to detailed documentation

Enhanced: .env.example

• Header explaining security requirements
• Required vs optional variables
• Direct link to setup guide

Context

The schema simplification changes (removing JSON Schema executor) are complete and verified via static analysis. Integration tests cannot execute in current environment due to COPILOT_AGENT_INJECTED_SECRET_NAMES not exposing actual secret values, but documentation now provides clear path for verification with proper API key access.

Original prompt

So we are currently defining a schema simply as a set of fields during plan with a custom format and then using an execute node to convert that to a json schema. We did that originally to have a schema we could use for structured output. But given all of our state updates are determinisitic and we never output a full state do we even need that anymore?

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