ci(plugins): check PluginKit ABI with Xcode's ABI checker

[datlechin]

Why

The PluginKit ABI check diffed .swiftinterface text against a merge base and ran in no workflow; a separate cadence script held the kit bump rule.

Change

• scripts/check-pluginkit-abi.sh [--frozen] [base] builds PluginKit at the newest v* tag and at HEAD with Xcode's ABI checker (RUN_SWIFT_ABI_CHECKER_TOOL). A breakage needs kit and minimum at tag kit + 1, a changed baseline kit + 1, an identical one the tag's kit.
• Passes -error-on-abi-breakage: without it swift-api-digester reports a breakage as a warning.
• macOS Tests runs it after AllPlugins, --frozen on a PR into release/*, with a full blob:none checkout for the tag.
• Deletes the cadence script and its test; verify.sh abi takes an optional base.

Verified

verify.sh abi against v0.82.0-beta.1 passes here with byte-identical baselines. In scratch trees a removed requirement fails, and passes at kit and minimum 40; an added defaulted requirement fails at kit 39 and passes at 40; --frozen fails on it.

Not in this PR

• detect-changed-paths.sh does not watch the script yet, so a PR touching only it skips the step.
• fix-issue/references/platform-apple.md still lists abi <merge-base>.