docs(v2): plan distributed societies on GKE Agent Sandbox

[chughtapan]

Summary

• Records the distributed-society ADR candidate around direct GKE Agent Sandbox resources, one application container per stable agent slot, exact generation-aware readiness, and a single dispatch.
• Preserves the prior Pod-attestation decision as superseded by Secret-bound slots, including the retained trust-boundary guarantees.
• Adds the local-first implementation plan and required fresh blind-review evidence.

Scope

This is documentation and decision evidence only. It adds no simulator, runtime, GKE, Temporal, or Kueue implementation code. The proposed implementation remains blocked on the simulator-handoff SHA and a separately admitted first-slice scope.

Validation

• Commit gate: documentation checks, architecture checks, lint, formatting, builds, test type checks, and guardrails.
• Fresh blind review: PASS after preserving the invalid and failed attempts and repairing the supersession lineage.

Admission

This draft awaits maintainer acceptance of the ADR review gate.

[chughtapan]

superseded bu #932