Fix inconsistencies between PowerShell snippets and JSON export examples in plan-export docs

[Copilot]

Two mismatches between the code snippets and their corresponding JSON output examples in the plan export documentation.

Changes

• docs/use/plan-export.md — Added -Actor 'HR-System' to the New-IdleRequest call so the snippet produces output that matches the JSON example showing "actor": "HR-System"
• docs/reference/specs/plan-export.md — Changed the plan.mode example from "PlanOnly" to null and clarified in the field table that mode is null when not explicitly set; aligns the spec with actual Export-IdlePlanObject behavior

# Before — actor not passed, but JSON example showed "actor": "HR-System"
$request = New-IdleRequest -LifecycleEvent 'Joiner' -CorrelationId (New-Guid) -IdentityKeys @{ ... }

# After — consistent with export output
$request = New-IdleRequest -LifecycleEvent 'Joiner' -CorrelationId (New-Guid) -Actor 'HR-System' -IdentityKeys @{ ... }

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