Migrate to mcdsl: auth, config, csrf, web

- Replace internal/auth with mcdsl/auth
- Replace internal/config with mcdsl/config (embed config.Base)
- Replace internal/webserver/csrf.go with mcdsl/csrf
- Use mcdsl/web for session cookies and template rendering
- Use mcdsl/httpserver for server setup and StatusWriter
- Remove direct mcias client library dependency
- Update .golangci.yaml to v2 format (formatters section)

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

mcat.toml

@@ -0,0 +1,13 @@
+[server]
+listen_addr = ":7443"
+tls_cert    = "/Users/kyle/tmp/ca/mc-localhost/cert.pem"
+tls_key     = "/Users/kyle/tmp/ca/mc-localhost/key.pem"
+
+[mcias]
+server_url   = "https://mcias.metacircular.net:8443"
+ca_cert      = "/Users/kyle/tmp/ca/ca.pem"
+service_name = "mcat"
+tags         = ["env:restricted"]
+
+[log]
+level = "info"