chore: add central probot settings.yml for default repository configuration (#5)

This PR adds the `.github/settings.yml` file to the central `.github`
repository. Once the Probot Settings GitHub App is installed, this
configuration will act as the global default for all new repositories in
the estate, enforcing standardized pull request and merge policies
automatically.

Author: hyperpolymath

.github/settings.yml

@@ -0,0 +1,16 @@
+repository:
+  has_issues: true
+  has_projects: true
+  has_wiki: true
+  default_branch: main
+  allow_squash_merge: true
+  allow_merge_commit: true
+  allow_rebase_merge: true
+  allow_auto_merge: true
+  delete_branch_on_merge: true
+  allow_update_branch: true
+  squash_merge_commit_title: PR_TITLE
+  squash_merge_commit_message: PR_BODY
+  merge_commit_title: PR_TITLE
+  merge_commit_message: PR_BODY
+  web_commit_signoff_required: true