Tidy Up: rename jdt2jar to jtd2jar #163

Problem

The schema-to-validator-JAR tool is misspelled throughout the repository as jdt2jar — it is jtd2jar for JSON Type Definition (RFC 8927). The misspelling appears in the module directory and artifactId, Java package/class names (json.java21.jdt2jar.Jdt2Jar), the GHCR Docker image name, the helper script, and all documentation.

Fix

Rename everything mechanically:

• Directory jdt2jar/ → jtd2jar/ (git mv, history preserved)
• Maven module/artifactId jdt2jar → jtd2jar
• Java package json.java21.jdt2jar → json.java21.jtd2jar, class Jdt2Jar → Jtd2Jar
• GHCR image ghcr.io/simbo1905/java.util.json.java21/jdt2jar → .../jtd2jar (in the release workflow and docs)
• Script scripts/jdt2jar.sh → scripts/jtd2jar.sh
• All references in README/AGENTS/index.html/workflows

Transition note

Docker image names are not aliased: the old jdt2jar tags remain on GHCR but will stop receiving updates; the next release publishes jtd2jar:<version> and jtd2jar:latest. The README gains a one-line note so existing consumers know to re-point.

Why

The typo is in user-facing published artifacts (GHCR image, release notes); anyone searching for "JTD" tooling will not find "jdt". Same mechanical rename discipline as the sandbox→incubator move.