Fix kubectl exec command: use -- separator and grep instead of Select-String

[Copilot]

Two bugs in the kubectl exec invocation caused runtime failures: a single - was used instead of -- to separate the pod name from the command (causing the container runtime to attempt executing - as a binary), and Select-String (PowerShell-only) was used in a Linux shell context.

Changes

• README: Documents the correct kubectl exec syntax and cross-platform filtering:

# Broken
kubectl exec -n contentautomation <pod> - printenv | Select-String "DB|POSTGRES|JDBC|SPRING_DATASOURCE"

# Fixed
kubectl exec -n contentautomation <pod> -- printenv | grep -E "DB|POSTGRES|JDBC|SPRING_DATASOURCE"

• Adds a quick-reference table mapping the incorrect forms to their correct equivalents

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