[sbglasius]

Moves the plugin onto Grails 8.0.0-M5, and brings the repo back up to date with
grails-plugin-template. Two commits, reviewable independently.

1. chore: migrate to Grails 8.0.0-M5

Grails 8.0.0-M5, Groovy 5.0.8, Spring Boot 4.1, Java 21, Gradle 9.6.0 — the toolchain
versions are taken from grails-core at v8.0.0-M5. CI derives its JDK from .sdkmanrc,
so no workflow change was needed.

Breakages found and fixed along the way:

• springDependencyManagement = false is deprecated in Grails 8 → bom = null.
• docs failed outright on Gradle 9: the root project's aggregateGroovyApiDoc resolved
  :greenmail's compile and groovy classpaths, which is no longer allowed across projects.
  The Groovydoc task now lives on the plugin project; the output layout is unchanged.
• jansi is no longer in the Grails BOM, so the unversioned dependency broke configuration
  of the example app.
• Both grails.mime blocks are gone. Grails 8 ships exactly the list they declared, and
  honours the Accept header for every user agent now.
• Logback 1.5 deprecated conversionRule converterClass → class.

Also fixes a pre-existing bug the migration surfaced: :app1:bootRun failed with
NoClassDefFoundError: grails.artefact.controller.RestResponder. The plugin's controller
carries the RestResponder trait, but the example app hand-lists Grails modules and omitted
grails-rest-transforms — only grails-testing-support-web supplied it, so the tests
passed while the app could not boot. This fails identically on 7.0.x; it is not a
migration regression.

2. chore: sync common files from grails-plugin-template

Replays the template's own files-sync.yml logic rather than hand-picking files.

Brings in verify-repository.groovy and the verify CI job that gates the build on it;
config.example-app applying config.code-coverage (so the example app's integration
tests now feed the aggregated coverage report); config.publish self-applying
config.project-metadata; config.publish-root taking the Maven group from a new
projectGroup property instead of hardcoding it; restored "do not edit directly" headers;
and robustness fixes in the update-versions / update-contributors scripts.

ci.yml also stops passing -PskipCodeStyle on the build-with-tests step, so CodeNarc
and Checkstyle now run in CI for the first time. Both are clean.

Two deliberate divergences from a literal sync:

• stale.yml → local-stale.yml. It is not a template workflow, and the workflow sync is
  an rsync --delete that spares only the local-*.yml names.
• gradle/ is not synced — it would downgrade the wrapper from 9.6.0 back to the
  template's 8.14.4.

One real breakage the sync caused

config.example-app pulling in config.code-coverage made :app1:integrationTest fail
with Expected configuration ':app1:jacocoAgent' to contain exactly one file, however, it contains no files. The jacoco plugin contributes its agent and ant coordinates through
defaultDependencies, which Gradle skips once a configuration has any dependency of its
own — and Grails 8 applies the Grails BOM to every declarable configuration,
jacocoAgent included, so the fallback never fires. The plugin project was unaffected only
because config.grails-plugin sets bom = null. config.code-coverage now declares the
coordinates outright.

Verification

Green from clean on JDK 21 / Gradle 9.6.0:

• ./gradlew build docs — 12 unit tests, 11 integration tests, CodeNarc and Checkstyle
• ./gradlew publishToMavenLocal — still publishes io.github.gpc:greenmail
• groovy .github/scripts/verify-repository.groovy — no errors
• :app1:bootRun on a real port, hitting /greenmail/list (200 text/html) and
  /greenmail/list.json (200 application/json)
• Booted once with spring-boot-properties-migrator attached: no warnings. It was removed
  again before this branch was committed.

Follow-ups, not in this PR

• Three fixes want upstreaming to grails-plugin-template: bom = null, the Gradle 9
  config.docs fix, and the jacoco defaultDependencies fix. Every template repo will hit
  all three the moment the template moves to Grails 8. The jacoco one is the most urgent.
• files-sync.yml rsyncs gradle/ unconditionally. It calls lock_excludes for
  .github/workflows, .github/scripts and build-logic, but not for gradle/ — so the
  .lock convention cannot protect the wrapper. Once greenmail is registered in the
  template's projects.yml, the first sync would silently downgrade its wrapper and break
  the build.
• The .lock files here pin what must not come from a Grails 7 template. All except
  CONTRIBUTING.md.lock are temporary and should be deleted once the template is on
  Grails 8.
• CONTRIBUTING.md links the licence as LICENSE, but the file is LICENSE.txt. The
  template's generic copy gets this right; ours is pinned, so it keeps the dead link.
• .agents/skills/enhance-plugin-with-template/ is stale here and arguably template-only —
  the sync neither updates nor removes it.