Add no-network core analyzer regression guard

[Will-thom]

Summary

• Add local-first regression tests for the core analyzer.
• Exercise core analysis with provider token environment variables unset.
• Add static guards against network/provider-client imports and provider environment reads in production core code.
• Document the guardrail in testing and permission posture docs.

Closes #80.

Validation

• go test ./...

[github-actions]

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