Add a fastlane pipeline that releases HealthChat to TestFlight (#2)

element-x-ios had no release machinery at all: no fastlane, no Gemfile, and none of
its workflows built or uploaded anything. Upstream ships via Xcode Cloud, which
needs Element's private Enterprise submodule and so is unavailable here, while the
legacy element-ios fork already proves a fastlane pipeline against the same Apple
team, certificate and App Store Connect key.

Tagging healthchat-v<version> now builds a signed App Store IPA on the org's Mac
Studio and uploads it to TestFlight. Release notes come from a new
HEALTHCHAT_CHANGES.md, because upstream's own tooling owns CHANGES.md.

The lane deliberately does not pin Xcode. The runner carries a single globally
selected version, and naming an absent one does not fail fast — it sends xcodes
into a multi-GB authenticated download that no CI secret can satisfy. Each run logs
xcodebuild -version instead, which is the only record of what compiled a build.

Everything here is fork-owned, since upstream has no fastlane directory, so none of
it can conflict on rebase.

.gitignore

@@ -49,3 +49,10 @@ test_output
 # Only commit Pkl packages (there may be other caches in here).
 Components/Secrets/vendor/*
 !Components/Secrets/vendor/package-2
+
+# fastlane release outputs
+out/
+provisioning_profiles/
+# Regenerated by every fastlane invocation, including on the runner. Ignored rather than
+# committed, matching element-ios.
+fastlane/README.md