fix(ci): prune snap assets from dev release

[pimlock]

Summary

Prune versioned Snap packages from the rolling dev release before publishing the current builds. This prevents Snap assets from accumulating while preserving the existing behavior for unmanaged assets.

Right now, the dev release has 370 artifacts on it: https://github.com/NVIDIA/OpenShell/releases/tag/dev

The ones that are not cleaned up are .snap.

Changes

• Treat .snap files with the openshell prefix as managed dev-release assets
• Update the missing-release log message to describe all managed asset pruning

Testing

• mise run pre-commit passes
• Unit tests added/updated (not applicable; workflow-only predicate change)
• E2E tests added/updated (not applicable)
• GitHub Actions workflow YAML parses successfully
• Managed-asset predicate accepts generated amd64/arm64 Snap filenames and rejects unmanaged assets
• git diff --check passes

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (not applicable; CI-only change)