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>

go.mod

@@ -0,0 +1,17 @@
+module git.wntrmute.dev/kyle/mcat
+
+go 1.25.7
+
+require (
+	git.wntrmute.dev/kyle/mcdsl v0.0.0
+	github.com/go-chi/chi/v5 v5.2.5
+	github.com/spf13/cobra v1.10.2
+)
+
+require (
+	github.com/inconshreveable/mousetrap v1.1.0 // indirect
+	github.com/pelletier/go-toml/v2 v2.3.0 // indirect
+	github.com/spf13/pflag v1.0.9 // indirect
+)
+
+replace git.wntrmute.dev/kyle/mcdsl => /home/kyle/src/metacircular/mcdsl