feat: add e2e tests, fix server shutdown and map race, update docs

- Add end-to-end test suite covering HTTP tunnel round-trip, POST
  forwarding, unknown tunnel 404, duplicate name rejection, unauthorized
  access, info endpoint, multi-tunnel routing, and graceful shutdown
- Fix server graceful shutdown by closing TCP listener on context cancel
- Fix data race in pkg/maps Entries() iterator by holding RLock
- Rewrite README with architecture, configuration, and usage docs
- Add AGENTS.md with project conventions and architecture guide
- Update flake.nix (add gopls) and flake.lock

pkg/maps/map.go

@@ -42,6 +42,8 @@ func (m *Map[K, V]) HasKey(key K) bool {
 
 func (m *Map[K, V]) Entries() iter.Seq2[K, V] {
 	return func(yield func(K, V) bool) {
+		m.mu.RLock()
+		defer m.mu.RUnlock()
 		for k, v := range m.items {
 			if !yield(k, v) {
 				return