Only install the release's attached zip when updating

[howarddc]

Problem

The update checker calls enableReleaseAssets() with the default PREFER_RELEASE_ASSETS behavior. If a release is ever published without everyalt-<version>.zip attached (or with a differently named file), it falls back to GitHub's auto-generated source zip. That zip doesn't contain vendor/plugin-update-checker (it's bundled by build.sh, not committed), so any site that installs it silently stops receiving updates.

Fix

• REQUIRE_RELEASE_ASSETS, so a release without a matching asset is skipped instead of falling back to the source zip.
• A name filter (/^everyalt-[0-9][0-9A-Za-z.\-]*\.zip$/), so only the built plugin zip counts as the asset.
• An "Unreleased" changelog entry in the README.

Same change as Site Manager 1.0.0 (howarddc/site-manager-plugin).

Testing

On a local WordPress 7.1 / PHP 8.5 site, using a zip built from this branch with its version lowered to 1.0.0:

• Update detected: WordPress offered 1.1.0, the latest published release (the 1.1.1 draft is correctly ignored), with download URL releases/download/v1.1.0/everyalt-1.1.0.zip.
• Update installed: wp plugin update everyalt installed it, and vendor/plugin-update-checker was still present afterwards.
• Missing asset skipped: with the name filter changed to match nothing (simulating a release without its zip), no update is offered. The old behavior would have offered GitHub's source zip.
• php -l passes and ./build.sh builds.

Note on rollout

The fix only protects sites once they're running a version that includes it. The current v1.1.1 draft was built from main before this change. Either merge this and rebuild and re-attach the 1.1.1 zip (moving the v1.1.1 tag), or ship it in 1.1.2.

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