Release Cleanup: Prevent release lane from running locally

[Copilot]

What?

Adds a hard CI-only guard to the release Fastlane lane so it cannot be run outside of a CI environment.

Why?

The release lane pushes a git tag to origin, uploads artifacts to the public S3 bucket, and creates a real GitHub Release. Running it locally by mistake would cause irreversible side effects. The lane already carried a **Don't run this locally** comment but had no enforcement.

How?

Added a UI.user_error! guard as the first statement in the release lane, consistent with the existing error-handling style throughout the Fastfile:

lane :release do |options|
  UI.user_error!('This lane must only run in CI. Use individual lanes (validate, update_swift_package, publish_to_s3, publish_release_to_github) for local diagnosis.') unless ENV['CI']
  ...
end

If ENV['CI'] is not set, the lane aborts immediately with a message pointing to the individual lanes suitable for local use.

Testing Instructions

1. Locally, run bundle exec fastlane release version:v0.0.0-test (without CI set in the environment).
2. Confirm the lane aborts with the error message and does not proceed to push any tag or upload any artifact.
3. Re-run with CI=true bundle exec fastlane release version:v0.0.0-test to confirm the guard passes and the lane continues normally (it will fail later on missing credentials/artifacts, which is expected).

Accessibility Testing Instructions

N/A — no UI changes.

Screenshots or screencast

[wpmobilebot]

XCFramework Build

This PR's XCFramework is available for testing. Add the following to your Package.swift:

.package(url: "https://github.com/wordpress-mobile/GutenbergKit", branch: "pr-build/517")

_{Built from c3a21bd}

[Copilot]

Pull request overview

Adds a CI-only guard to the Fastlane release lane to prevent accidental local execution of release publishing steps.

Changes:

• Aborts release immediately when ENV['CI'] is not set.
• Keeps existing release orchestration unchanged for CI runs.

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[jkmassel]

Thanks for taking care of this followup!