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

server/server.go

@@ -70,11 +70,23 @@ func (s *Server) Start() error {
 	}
 	defer listener.Close()
 
+	// Context Cancellation - Close the listener when the context is cancelled
+	// so that Accept() unblocks and the loop exits cleanly.
+	go func() {
+		<-s.ctx.Done()
+		listener.Close()
+	}()
+
 	// Start Listening
 	log.Infof("conduit server listening on %s", s.cfg.BindAddress)
 	for {
 		conn, err := listener.Accept()
 		if err != nil {
+			// Expected Error on Shutdown
+			if s.ctx.Err() != nil {
+				log.Info("conduit server shutting down")
+				return nil
+			}
 			log.WithError(err).Error("error accepting connection")
 			continue
 		}