Dual-space blackboard — private agent scratchpad separate from shared workspace

[mdproctor]

Overview

Add a private scratchpad zone to the blackboard so agents can work internally before committing results to the shared workspace.

Part of epic #77 (Blackboard Architecture Evolution).

Background

Recent LLM-based blackboard research (arXiv 2507.01701) introduces a dual-space architecture: a public shared zone (the current CaseFile) and a private zone per agent. Agents use the private space for internal deliberation, partial computations, and "debate" steps — only committing final results to the public blackboard.

Benefits:

• Reduces noise in the shared workspace (no partial/intermediate writes visible to other agents)
• Enables agent-internal multi-step reasoning without polluting shared state
• Allows revision before commitment (optimistic internal iteration)

What to build

• AgentScratchpad — a per-activation private key-value store, scoped to a single PlanItem execution
• Passed to TaskDefinition.execute() alongside the CaseFile
• Contents are never visible to other TaskDefinitions
• Agent explicitly calls scratchpad.commit(key, value) to promote to shared CaseFile
• Scratchpad is discarded (not persisted) when activation completes

Relationship to current work

Depends on casehub-blackboard/ module (issue #76). The AgentScratchpad would be a casehub-blackboard/ concern, not a core engine concern.