🐛 fix artifact identity collisions

[clubanderson]

Summary

• mint timestamp IDs with a random suffix so new artifact names are globally unique across branches
• expose artifact_id in named artifact status payloads for orchestrator join keys
• keep counter-prefixed artifacts valid as timestamp-mode aliases for migration

Fixes #48

Tests

• go test ./... -count=1

[kubestellar-prow]

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