Support dynamic roles in initiatives and skill freshness

[grafkest]

Summary

• surface roles from the skill registry in initiative planning so newly added skills and roles appear in work assignments
• capture skill creation timestamps when adding experts manually or via Excel import and use them when deriving freshness
• expose known roles from the skill registry for downstream consumers

Testing

• npm run build (fails: Rollup cannot resolve optional dependency xlsx during Vite build)

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Codex Task