feat: layout sidecar file support

[intel352]

Summary

• Read/write .workflow-editor.json sidecar files for editor layout positions
• Layout is no longer embedded in the YAML config, preventing config pollution
• JBCefJSQuery bridge for window.hostBridge.saveLayout() callback

Design

See: workflow-editor docs/plans/2026-03-17-editor-engine-contract-design.md

Test plan

• Open a YAML file in the visual editor, move nodes around
• Verify .workflow-editor.json is created alongside the YAML
• Re-open the file — positions should be restored from sidecar
• Verify the YAML file is NOT modified with ui_position fields

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