Add a root `release` task that bundles every plugin jar into a clean release/ folder

Add a root Gradle release task so a single command produces a clean release/ folder containing the shaded jar of every deployable Paper plugin.

What it does

• ./gradlew release builds the shadowJar of :treasury, :business, :treasury-api-plugin, :chestshop and stages them into release/ at the repo root.
• Implemented as a Sync (not Copy), so the destination is mirrored to exactly the current jars — stale artifacts from a prior run are removed, giving a clean folder every time.
• treasury-rest-api is excluded by design (ships as a container image, not a plugin).
• release/ is already git-ignored via the existing *.jar rule.

Wiki (wiki/build-and-test.md) updated to document the task.