chore(release): set up tag-driven HealthChat release pipeline

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.

HEALTHCHAT_CHANGES.md

@@ -0,0 +1,16 @@
+# HealthChat Changelog
+
+Release notes for HealthChat builds of element-web. The release pipeline
+(`.github/workflows/build-healthchat.yaml`) extracts the matching `## Changes
+in <version>` section and uses it as the body of the GitHub Release.
+
+One section per released version, newest on top. Upstream element-hq
+`CHANGELOG.md` is kept separate and untouched.
+
+## Changes in 1.0.0 (2026-04-21)
+
+First tagged HealthChat release of element-web.
+
+🙌 Improvements
+
+- Initial HealthChat branding, theming, and healthchat-workspace module integration.