Initial commit: plugin skeleton, compiles against CoreDNS 1.14.3

Sets up the package layout for a CoreDNS plugin that will accept RFC 2136
dynamic updates with TSIG authentication, primarily targeting self-hosted
ACME DNS-01 cert automation.

What this commit gives us:
- go.mod against coredns/caddy v1.1.4, coredns/coredns v1.14.3, miekg/dns v1.1.72
- plugin.go: RFC2136 struct + Handler interface (ServeDNS is pass-through)
- setup.go: init() registration + Corefile parser (skeleton — recognizes
  tsig-key, ttl, persist directives but doesn't yet wire them)
- README.md, .gitignore

go build ./... clean. No tests yet — those come with Phase 1.2 alongside
the actual UPDATE handler and in-memory store.

Plan: ~/.claude/plans/dood-does-coredns-offer-enumerated-piglet.md

.gitignore

@@ -0,0 +1,26 @@
+# Go build artifacts
+*.exe
+*.exe~
+*.dll
+*.so
+*.dylib
+*.test
+*.out
+
+# Dependency directories (modules use go.sum, not vendor/)
+vendor/
+
+# IDE / editor
+.vscode/
+.idea/
+*.swp
+*.swo
+.DS_Store
+
+# Local development
+*.local.go
+coverage.out
+coverage.html
+
+# Built CoreDNS binary if someone runs `make` from here for testing
+/coredns