Add TOML config file support to mcrctl

Loads defaults from ~/.config/mcrctl.toml (or XDG_CONFIG_HOME).
Resolution order: flag > env (MCR_TOKEN) > config file.
Adds --config flag to specify an explicit path.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

deploy/examples/mcrctl.toml

@@ -0,0 +1,7 @@
+# mcrctl — Metacircular Container Registry CLI configuration.
+# Copy to ~/.config/mcrctl.toml and edit.
+
+server  = "https://registry.example.com:8443"   # REST API base URL
+grpc    = "registry.example.com:9443"            # gRPC server address (optional)
+token   = ""                                     # bearer token (MCR_TOKEN env overrides)
+ca_cert = ""                                     # custom CA certificate PEM file