Remove redundant guidance to read .env directly - #801

Thanks to the recent core-guideline cleanup work in #722 and #787 — the file reads cleanly. I'd like to propose removing one more redundant line: the instruction to read .env directly to check environment variables.

The line immediately above already documents config:show and direct reads of config/*.php, which cover the same use case. After tracing this through, the .env line looks redundant for legitimate lookups while pointing the model at a file Anthropic's secure-deployment guide and settings reference treat as a sensitive denylist target alongside ~/.aws/credentials and *.pem.

I saw the prior discussion in #728 and agree with the response there: guideline text can't enforce access restrictions, that belongs in the agent's permission layer. This PR isn't trying to revisit that. It's a removal in the same spirit as #787, where a section was deleted once an adjacent mechanism made it redundant.

Behavior for users workflow is unchanged: agents can still read .env files.