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
- CI: trigger on decoupling branch, use dynamic ref_name

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

.gitmodules

@@ -1,3 +1,4 @@
-[submodule "shared_lib"]
-	path = shared_lib
+[submodule "shared"]
+	path = shared
 	url = https://git.rose-ash.com/coop/shared.git
+	branch = decoupling