Add a skill for releasing HealthChat to TestFlight (#3)

Documents the release process this fork actually uses: tagging, the numeric-only
version rule, the five required secrets, the runner's hand-installed toolchain,
certificate renewal, and promoting a TestFlight build to the App Store. Modelled on
the legacy element-ios skill, corrected wherever this fork differs — notably that
there is no Xcode pin and no Brewfile, and that only five secrets exist rather than
the six the legacy skill lists.

The section with no upstream equivalent is the migration contract. CLASSIC_APP_*
and the .requiresBackup fall-through are load-bearing for every existing user and
fail silently, so a well-meaning cleanup back to upstream's values would sign
everyone out with nothing in CI to catch it.

AGENTS.md delegates to the skill rather than duplicating the runbook.

AGENTS.md

@@ -67,6 +67,14 @@ PRs must follow rules. Prefer Xcode MCP tools over terminal commands.
 
 ---
 
+## Releasing
+
+HealthChat ship via tag `healthchat-v*` → TestFlight. Full runbook in
+`.agents/skills/releasing-healthchat/SKILL.md`. Read skill before release, cert
+renewal, or touching `fastlane/`. No duplicate runbook here.
+
+---
+
 ## Project Structure
 
 ### Build System