Replace shared_lib submodule with shared for decoupling deploy

- Swap shared_lib submodule → shared (tracking decoupling branch)
- Dockerfile: shared_lib/ → shared/, remove bp symlink hack
- entrypoint.sh: cd shared for alembic upgrade head
- CI: trigger on decoupling branch, use dynamic ref_name

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

entrypoint.sh

@@ -13,7 +13,7 @@ fi
 # Run DB migrations only if RUN_MIGRATIONS=true (blog service only)
 if [[ "${RUN_MIGRATIONS:-}" == "true" ]]; then
   echo "Running Alembic migrations..."
-  cd shared_lib && alembic upgrade head && cd /app
+  cd shared && alembic upgrade head && cd /app
 fi
 
 # Clear Redis page cache on deploy