Stateful resources server checkpoint adapters #3219

Parent issue

Sub-issue of #3024.

Problem

Stateful resources servers retain mutable session state between agent operations. Generic checkpoint infrastructure can fence mutations and serialize session identity, but each environment must explicitly export and restore the state it owns. Inheriting a continuation claim without a proven codec can silently restore an invalid execution.

Scope

Track logical-state adapters for resources servers that can serialize or deterministically reconstruct their sessions. Each child must identify the state owner, payload schema, mutation routes, resource revision behavior, restore atomicity, terminal cleanup, and unsupported effects.

Sandbox/provider-backed state, grouped drain-only services, and already-adapted examples are tracked separately.

Shared acceptance criteria

• Sessions are keyed by stable rollout and attempt identity.
• Mutations are fenced and revisions advance only after success.
• Export waits for accepted session mutations and activates no partial restore.
• Restore installs attempt N+1 while stale attempt N is rejected.
• Agent boundaries reconcile the exact prepared resource revision.
• Terminal retirement and failed verification preserve the intended state.
• Environment-specific next-operation equivalence is tested after process replacement.