Set up Datadog sourcemap upload via the build plugin

Adds build-time sourcemap upload to a frontend project using the Datadog build plugin. Sourcemaps are uploaded as part of the build, so every deploy ships un-minified stack traces to RUM and Error Tracking. Git metadata is attached automatically, which links errors back to source files.

Follow steps 1-7 in order.

Before starting any work: Create exactly 7 todo items — one per step below. Do not begin implementation until all 7 todos are created. Do not compress or merge steps.

Never invent a value. Three fields (service, releaseVersion, minifiedPathPrefix) must match how the app is actually deployed. If you cannot read a value from the repo, stop and ask the user. Do not write a placeholder, a TODO, or a guessed hostname.

Scope