GRM-20: fix: set runner_mode to binary in multi-instance converge

The multi-instance scenario verify playbook expects config files at
/etc/gitea-runner/<name>/config.yaml (binary mode path). Without
runner_mode set, the role defaulted to docker mode, which places the
config in /var/lib/gitea-runner/<name>/config.yaml instead.

Add runner_mode: binary to both converge plays so config placement
matches the verify assertions.

ansible/roles/gitea-runner/molecule/multi-instance/converge.yml

@@ -6,6 +6,7 @@
     gitea_url: "http://localhost:3000"
     registration_token: "fake-token-for-testing"
     runner_name: "molecule-runner-a"
+    runner_mode: "binary"
     skip_runner_registration: true
   roles:
     - role: gitea-runner
@@ -17,6 +18,7 @@
     gitea_url: "http://localhost:3000"
     registration_token: "fake-token-for-testing"
     runner_name: "molecule-runner-b"
+    runner_mode: "binary"
     skip_runner_registration: true
   roles:
     - role: gitea-runner