Add mise configuration for local toolchain management

Sets up `mise` for automatic Ruby provisioning, matching the pattern
already in use in minfraud-api-dotnet and GeoIP2-dotnet. Uses `latest`
to let contributors pick up the newest stable Ruby, mirroring the
behavior of the dotnet repos.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: oschwald

mise.lock

@@ -0,0 +1,17 @@
+[settings]
+experimental = true
+lockfile = true
+disable_backends = [
+    "asdf",
+    "vfox",
+]
+
+[tools]
+ruby = "latest"
+
+[hooks]
+enter = "mise install --quiet --locked"
+
+[[watch_files]]
+patterns = ["mise.toml", "mise.lock"]
+run = "mise install --quiet --locked"