qol-sweep: create registry manifest for workflow-plugin-steam

[intel352]

QoL sweep 2026-05-19 added experimental banner + MIT LICENSE to workflow-plugin-steam but the plugin has no entry in workflow-registry/plugins/. Follow-up to create a manifest.json so this plugin appears in wfctl marketplace and gets a published status field.

Plugin GitHub repo: https://github.com/GoCodeAlone/workflow-plugin-steam

Default status when manifest is created: experimental (no merged GoCodeAlone production usage as of 2026-05-19).

See: workflow#714 (umbrella sweep), workflow#717 (deferred-work tracker).

[intel352]

Verified on current origin/main: plugins/steam/manifest.json exists and has populated capabilities. Closing as completed.