feat: share release notifications for Homebrew and Nix #820

Problem

Homebrew has a release-completion notification workflow, while the new nix-mcpp repository already accepts the same mcpp-release dispatch but is not notified upstream. Copying workflows would duplicate version resolution and notification logic. Resolving a version from workflow_run.head_sha also selects the wrong release when release is manually dispatched to rebuild another tag.

Proposed change

Publish the sealed release manifest as a run artifact, and consume that exact artifact in a downstream publish-packages workflow. Notify Homebrew and Nix using a shared matrix with independent failures and channel-specific credentials. Preserve the PyPI and AUR workflows, which have distinct validation and publishing contracts. Support manual notification of an exact release tag and selected channel. Report notification acceptance separately from downstream publication.

Validation

Add focused tests for manifest identity validation and dispatch behavior, run workflow assertion checks and actionlint, and run relevant script tests. No external package publication is required to validate the change.