docs: introduce the Sentinel Operations Graph (SOG)

[CodeBuildder]

Summary

Standardizes Sentinel Operations Graph terminology and turns the Argus README into a paved two-path setup for reviewers.

Changes

• replaces legacy world-model naming with Sentinel Operations Graph (SOG)
• adds a clear Start here decision between local synthetic and live k3s
• makes the read-only cluster preflight the required first live step
• explains the OrbStack built-in context versus the real three-node argus context
• keeps OpenAI Responses API configuration and quota requirements explicit

Verification

• make test
• shell syntax checks for local and cluster demo scripts
• tracked provider-term scan is clean
• git diff --check

Closes #105
Closes #107