appgate_upload no longer requires app_id. Without it the action calls
POST /api/releases with the org key (agk_...) and lets the server find
or create the app by bundle_id + platform. Platform is detected from the
file extension (or set via `platform`), bundle id is read with the
app-info gem (or set via `bundle_id`); `app_name` is passed through.
A 409 lists the matching app ids and asks for app_id. The app_id path
is unchanged. Status polling now surfaces the server's `error` field.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Set APPGATE_LINK and APPGATE_LINK_<PLATFORM> on upload and persist them
to a same-pipeline cache file, restoring into ENV on plugin startup so
later fastlane runs or CI jobs on the same checkout (e.g. notify_slack)
can read the link without any Fastfile or CI changes.

Capture the share_url returned by the release creation endpoint and
expose it as ENV['APPGATE_LINK'], lane_context[SharedValues::APPGATE_LINK],
and the action's return value, so later lane steps (e.g. Slack) can
reference the public download page. Warn when older servers omit it.