build: gitignore .claude/worktrees

Claude Code agent worktrees are embedded git repos under .claude/worktrees;
ignore them so `git add -A` can't sweep them (or stray demo files) into a
commit.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: dfa1

.gitignore

@@ -7,3 +7,6 @@ target/
 dependency-reduced-pom.xml
 release.properties
 *.releaseBackup
+
+# Claude Code agent worktrees (embedded git repos; never part of the project)
+.claude/worktrees/