Step-level replay: memoized agent results

[Michaelliv]

True resume that skips completed agents.

Approach:

• Hash script body
• Key agent results by (scriptHash, phase, label)
• Persist results under workflow-runs//results/.json
• On resume, runWorkflow short-circuits agent() calls with a memoized result

Requires stable (phase, label) per agent. Document determinism rules accordingly.

Parent: #1