Define external scheduler path for offline-agent and NPC simulation

[JOY]

Project: Second Spawn
Area: AI Agent
Milestone: Foundation
Priority: P2
Size: M
Review Gate: Local Review
Verification: Docs Lint

Goal

Keep long-running AI simulation out of Nakama request handlers while still using Nakama as durable game backend state.

Scope

• Decide whether periodic simulation runs in the Fusion dedicated server, a worker process, or a scheduler that calls bounded Nakama RPCs.
• Define batch size, cooldowns, failure behavior, and observability.
• Ensure offline agents and NPCs still emit validated intents rather than direct game mutations.
• Keep LLM budgets and activity logs tied back to Nakama identity.

Acceptance Criteria

• Docs define the first scheduler architecture for autonomous NPC/offline-agent sweeps.
• The design preserves Fusion authority and Nakama validation.
• Implementation follow-up is clear and testable.