Upgrade spotless-maven-plugin to 3.x to support git worktrees #2665

Task summary

mvn spotless:check and mvn spotless:apply fail in a checkout created with git worktree add:

Execution default-cli of goal com.diffplug.spotless:spotless-maven-plugin:2.44.5:check failed: Cannot find git repository in any parent directory

In a worktree .git is a file that points to the main repository, and the JGit version bundled with spotless-maven-plugin 2.44.5 cannot follow it. Because of <ratchetFrom> in the root pom.xml, Spotless needs git and so the format tier of the development harness (AGENTS.md, step 4) cannot run in a worktree at all. Worktrees are a common way to work on several issues in parallel, and the contribute skill of #2591 recommends them.

This is a known Spotless issue (diffplug/spotless#1765), fixed in spotless-maven-plugin 3.0.0 by the upgrade to JGit 7.3.

Proposal: upgrade spotless-maven-plugin in the root pom.xml from 2.44.5 to the latest 3.x (3.10.4 at the time of writing).

Things to check in the PR:

• Breaking changes of 3.0.0: Java 17 is required (fine, IDEasy builds with Java 25), removeWildcardImports was renamed to forbidWildcardImports (not used in our configuration), the git pre-push hook location changed (not used, we have git-hooks/pre-commit).
• spotless:check on main and on a branch with Java changes gives the same result as before, so that the upgrade does not suddenly flag or reformat existing code.
• The Check formatting step of .github/workflows/build-pr.yml still passes.
• Remove the worktree hint from AGENTS.md and the contribute skill that Share agent skills in the repository and close gaps in AGENTS.md #2591 adds.

A quick local check: with the version changed to 3.10.4, mvn spotless:check -pl cli -Dspotless.ratchetFrom=upstream/main succeeds in a worktree, while 2.44.5 fails with the error above.

Additional context

• Found while working on Share agent skills in the repository and close gaps in AGENTS.md #2591 (PR #2591: share agent skills in the repository and close gaps in AGENTS.md #2664).
• Spotless changelog: https://github.com/diffplug/spotless/blob/main/plugin-maven/CHANGES.md (section 3.0.0)