Symlink the shared requirements into each Molecule scenario

Molecule checks for a requirements file at the scenario's default path
before it installs anything, and warns "Missing roles requirements file"
when it is absent - so `requirements-file` pointing elsewhere was silently
ignored and the galaxy roles were never installed. CI failed on a missing
ansible-role-docker; the local run passed only because those roles were
already in ~/.ansible/roles from earlier work, which is exactly the kind of
difference a clean CI run exists to expose.

A symlink satisfies Molecule's check while keeping one copy of the pins.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

roles/custom/matrix-alertmanager-receiver/molecule/default/molecule.yml

@@ -6,7 +6,7 @@
 dependency:
   name: galaxy
   options:
-    requirements-file: ../../../../../molecule-shared/requirements.yml
+    requirements-file: requirements.yml
     force: true
 driver:
   name: docker

roles/custom/matrix-alertmanager-receiver/molecule/default/requirements.yml

@@ -0,0 +1 @@
+../../../../../molecule-shared/requirements.yml