fix(openclaw): use ADC instead of service account keys (#207)

[Sabina8205]

Summary

Updates the OpenClaw Agent Platform migration guidance to use secure Application Default Credentials (ADC) instead of downloaded service-account keys.

Changes

• Removes service-account key creation and copying instructions.
• Removes guidance to disable the organization policy that blocks key creation.
• Uses the attached service account automatically on Compute Engine VMs.
• Documents gcloud auth application-default login for workstations.
• Recommends Workload Identity Federation for non-Google-Cloud hosts.
• Removes GOOGLE_APPLICATION_CREDENTIALS from the OpenClaw configuration example.
• Removes the no-longer-needed project-number lookup, including its incorrect VM metadata endpoint.

Validation

• Ran git diff --check.

Fixes #207

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