chore: standardize YAML file extensions

Use .yml consistently across role metadata and tooling configuration.

Co-authored-by: Copilot <copilot@github.com>

default/main.yml

@@ -0,0 +1,18 @@
+---
+
+root: {}
+#   password: ""
+#   ssh:
+#     config:
+#     - Host: "*"
+#       StrictHostKeyChecking: "no"
+#       UserKnownHostFile: /dev/null
+#     authorized_keys:
+#     - content: ssh-ed25519 AAAAC3NzaC... alice
+#       # command: "" # optional
+#     private_keys:
+#     - filename: alice@alice-pc.ed25519.key
+#       content: |
+#         -----BEGIN OPENSSH PRIVATE KEY-----
+#         ...
+#         -----END OPENSSH PRIVATE KEY-----