COR-181 - Make MailCore2 independent from AWS

[dbezverkhnii]

Note

Medium Risk
Changes how all Windows Spark consumers fetch MailCore C++ binaries; a missing or mismatched release asset breaks Windows builds until someone publishes from a Windows machine with gh auth.

Overview
Replaces the Spark Windows MailCore prebuilt pipeline so artifacts and binary deps no longer use the spark-prebuilt-binaries S3 bucket or SPARK_PREBUILT_KEY. Prebuilts are mailcore2-windows-<digest>.zip on the permanent windows-prebuilt GitHub release; openssl/sasl/zlib come from mailcore2-windows-deps-1.zip on the same release.

The digest is derived from git tree entries over src (excluding src/swift), CMakeLists.txt, and build-windows-5.10, so checkouts pick the right archive without bumping $PrebuiltMailcoreVersion in Get-Mailcore2.ps1. Publish-Mailcore2Prebuilt.ps1 builds, stamps etc/mailcore2-source-digest, verifies the package, and uploads via gh. Get-Mailcore2.ps1 downloads publicly, treats only HTTP 404 as “missing prebuilt”, and rejects archives whose stamped digest does not match.

A new pull-request job (mailcore2 - Windows prebuilt on Ubuntu) runs Check-PrebuiltPublished.ps1 against the merge commit so a forgotten publish fails in the PR (including rebase hints when the branch head already has an archive). README.md and new AGENTS.md document publish and check behavior.

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