ci: replace Maven Checkstyle with Flint

[zeitlinger]

Summary

• use the released Flint 0.22.10 Checkstyle implementation
• add Checkstyle 13.8.0 as the direct checkstyle mise entry
• update CI to mise 2026.7.15 so that direct registry entry is available in every workflow
• allow the verified @yarnpkg/libzip 3.2.2 release for Renovate because it was published without provenance
• remove the Maven Checkstyle plugin and its obsolete skip settings so Checkstyle runs only once, through Flint
• preserve the repository's previous main-source scope with narrow suppressions for test and archived Java sources
• suppress VariableDeclarationUsageDistance for examples, where setup declarations intentionally precede simulated work

Flint Checkstyle support is released in v0.22.10, and this PR uses the standard aqua:grafana/flint backend for Flint itself.

Testing

• mise exec -- flint run --full checkstyle
• mise run lint:fix
• mise run build

[zeitlinger]

This now removes the Maven Checkstyle plugin to avoid running Checkstyle twice. The PR uses the Aqua package from aquaproj/aqua-registry#57625; CI installation depends on that registry PR merging.

[zeitlinger]

@copilot resolve the merge conflicts in this pull request