Flatten the :chestshop:plugin nesting into a single :chestshop module

ChestShop was the only project whose code lived in a nested subproject (:chestshop:plugin) under an aggregator (chestshop/build.gradle.kts held only a subprojects{} repos block + build-metadata extra props) — a leftover from the old multi-module Maven build. Flatten it to match treasury/business: code at chestshop/src, one chestshop/build.gradle.kts, project path :chestshop.

• chestshop/plugin/src → chestshop/src; merge the aggregator + module build files into one standalone module build (repos, toolchain, build-metadata vals, deps, processResources, jacoco, shadowJar); project(":chestshop").projectDir → projectDir.
• settings.gradle.kts: drop include(":chestshop:plugin").
• .github/workflows/chestshop-test.yml: :chestshop:plugin:* → :chestshop:* and chestshop/plugin/build/* report paths → chestshop/build/*.
• Docs updated (root build comment, CLAUDE.md, README.md, wiki/build-and-test.md, wiki/ci-cd.md).
• Also removed the dead install_dependency_to_repo.sh (it mvn install-ed into the deleted vendored chestshop/repo).

Verified by a clean :chestshop:build (compile + tests + check + shadowJar → ChestShop.jar).