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.lock

@@ -20,11 +20,11 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1760038930,
-        "narHash": "sha256-Oncbh0UmHjSlxO7ErQDM3KM0A5/Znfofj2BSzlHLeVw=",
+        "lastModified": 1777578337,
+        "narHash": "sha256-Ad49moKWeXtKBJNy2ebiTQUEgdLyvGmTeykAQ9xM+Z4=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "0b4defa2584313f3b781240b29d61f6f9f7e0df3",
+        "rev": "15f4ee454b1dce334612fa6843b3e05cf546efab",
         "type": "github"
       },
       "original": {