Configure monorepo build: unified CI, local deps, .dockerignore

- Dockerfiles use monorepo root as build context
- common/ and core/ installed as local packages (no git+https)
- Client tarball built from local client/ dir
- Unified CI with change detection: common/core -> rebuild both
- Per-repo CI workflows removed

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

l2/requirements.txt

@@ -9,5 +9,4 @@ markdown>=3.5.0
 python-multipart>=0.0.6
 asyncpg>=0.29.0
 boto3>=1.34.0
-# Shared components
-git+https://git.rose-ash.com/art-dag/common.git@889ea98
+# common (artdag_common) installed from local dir in Dockerfile