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

@@ -18,9 +18,22 @@ DerivedData/
 .swiftpm
 .netrc
 
+# fastlane
+#
+# It is recommended to not store the screenshots in the git repo.
+# Instead, use fastlane to re-generate the screenshots whenever they are needed.
+# For more information about the recommended setup visit:
+# https://docs.fastlane.tools/best-practices/source-control/#source-control
+
+fastlane/report.xml
+fastlane/Preview.html
+fastlane/screenshots/**/*.png
+fastlane/test_output
+towncrier.toml.tmp
+version_changes.md
+
 build
 /vendor/
-*/xcshareddata/xcodecloud
 
 ## macOS Files
 .DS_Store
@@ -37,5 +50,9 @@ test_output
 Components/Secrets/vendor/*
 !Components/Secrets/vendor/package-2
 
-# Local SDK development override
-matrix-rust-components-swift
+# 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