run-acceptance-tests: add environment variable setup guidance

[AdamTylerLynch]

The runbook's step 2 said only "suggest how to set up these environment variables
securely" — no discovery method, no handling guidance. Now it is concrete while
keeping the skill a tight runbook:

• Discover required variables from PreCheck/testAccPreCheck and
  grep -rn "os.Getenv" --include="*_test.go", plus README/CONTRIBUTING/.env.example
• Set variables per test invocation rather than exporting into the shell profile;
  never write secrets into files inside the repository
• Warn before running that acceptance tests create real, possibly billable
  infrastructure; confirm credentials target a test account
• Always pass an explicit -timeout (go test kills runs at 10 minutes by default;
  acceptance tests routinely exceed it)
• Interrupted runs may leak resources — run sweepers if the provider registers them
• Typo fixes ("focussed", "persistance") and soft cross-references to
  provider-test-patterns and provider-configuration

Verification: scripts/validate-structure.sh passes; single-file diff.