fix(deploy): the images drop third_party/'s overrides instead of copying it

Two deploys failed building the images (4 Nov 2026): first the
dictionaries' image lacked third_party/ (pub refuses an override whose
path is missing, exit 66), then, with it copied, pub refused the
overrides themselves, as the three plugins depend on the Flutter SDK,
which the dart image doesn't have (exit 69); every image would have.
No image's package uses them, so tool/workspace_only.sh, which already
narrows the root pubspec for an image, drops the overrides naming
third_party/, and no Dockerfile copies it.

Each image's build stage replayed from this commit with a standalone
Dart 3.13.4, a fresh pub cache, dash and mawk: all four build, and the
commit deployed before fails as the deploy did.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/train/halkyon_train_server/Dockerfile

@@ -12,9 +12,6 @@ FROM dart:3.13.4 AS build
 WORKDIR /app
 COPY pubspec.yaml pubspec.lock ./
 COPY tool/workspace_only.sh tool/
-# The root's dependency_overrides name these by path; pub refuses a path
-# that isn't there, though no server uses them.
-COPY third_party third_party
 COPY apps/train/halkyon_train_server apps/train/halkyon_train_server
 COPY packages/halkyon_base_server packages/halkyon_base_server
 COPY packages/halkyon_base_client packages/halkyon_base_client