Convert build-healthchat workflow from branch-push to tag-push trigger
(healthchat-v*), add version/workspace-ref/release-notes resolution,
and create a GitHub Release on tag push. Pin healthchat-workspace to
v0.5.0-rc.4 via .healthchat-workspace-version, seed the changelog with
1.0.0, and document the runbook in the releasing-healthchat skill.