Add Engineering Workflow plugin branding

[xeonvs]

Summary

• add the approved opaque workflow-cycle SVG and deterministic PNG assets
• expose Codex brandColor, composerIcon, logo, and logoDark metadata
• release synchronized source/package version 0.9.4 without changing runtime contracts

Validation

• scripts/dev_check.py full: 9/9
• scripts/dev_check.py security: 3/3
• skill and Codex plugin validators
• Claude plugin and marketplace strict validators
• full-size and compact visual QA; all PNGs are opaque RGB