Rewrite sbt-riddl plugin with auto-acquisition and full CLI

Replace the minimal sbt-riddl plugin with a full-featured version
that auto-downloads riddlc from GitHub releases, supports batch
operations over multiple .conf files, and provides pre-compile
validation.

Key changes:
- Auto-download riddlc with platform detection and caching
  in ~/.cache/riddlc/<version>/
- Three-tier binary resolution: explicit path > download > PATH
- New tasks: riddlcDownload, riddlcBinary, riddlcValidate,
  riddlcParse, riddlcBastify, riddlcPrettify, riddlcInfo,
  riddlcShowVersion
- Batch .conf scanning with configurable exclusions
- Pre-compile validation hook (riddlcValidateOnCompile)
- riddlc() curried helper for one-line project configuration
- Remove obsolete hugo command, V.scala, findRiddlcTask
- Update scripted test for new plugin API

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: reid-spencer