ci(plugins): check the PluginKit minimum and test the ABI verdict

[datlechin]

Why

Follow-up to #3552. check-pluginkit-abi.sh dropped the base's TABLEPRO_PLUGINKIT_MINIMUM_VERSION, so a PR could lower the minimum with no ABI change, or keep a reverted break's minimum above the kit, which traps the compatibleKits range in PluginManager. An xcconfig it could not parse failed the step with no message.

Change

• The minimum must equal the base's unless the checker reports a breakage, --frozen included. The failure names both settings.
• An unreadable Configs/PluginKit.xcconfig fails with an ::error::.
• scripts/ci/test_check_pluginkit_abi.py covers each verdict, --frozen and the default base against a stub xcodebuild, in Repo Hygiene.
• plugin-system.md points to ci.md for when CI runs the check; the fix-issue abi row takes [base].

Verified

• The minimum and xcconfig cases fail on main's script and pass here.
• verify.sh abi on Xcode 27 passes as unchanged and fails with the minimum at 18.

Not in this PR

• The detectors that watch the script: ci: run macOS Tests when the PluginKit ABI checker changes #3560.