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

flake.nix

@@ -6,8 +6,14 @@
     flake-utils.url = "github:numtide/flake-utils";
   };
 
-  outputs = { self, nixpkgs, flake-utils }:
-    flake-utils.lib.eachDefaultSystem (system:
+  outputs =
+    { self
+    , nixpkgs
+    , flake-utils
+    ,
+    }:
+    flake-utils.lib.eachDefaultSystem (
+      system:
       let
         pkgs = nixpkgs.legacyPackages.${system};
       in
@@ -15,6 +21,7 @@
         devShells.default = pkgs.mkShell {
           packages = with pkgs; [
             go
+            gopls
             golangci-lint
           ];
           shellHook = ''