Remove the Go reference server; keep only Rust

The Go implementation is preserved in the v1.0.0 tag. Its observable
behaviour stays the API reference: responses recorded from the Go server
live in tools/compatibility_expected.txt, and tools/compatibility.py now
compares the Rust server against them (--update re-records after an
intentional change). The Go SQLite schema is frozen in
tests/fixtures/go_schema_v1.sql for the existing-database test.

Move the web client from internal/httpapi/web to web/, drop the Go CI job
and Go version pin, and update README, AGENTS.md and docs.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/service-worker.js

@@ -0,0 +1,22 @@
+const CACHE = "balance-shell-v2";
+const SHELL = ["/", "/styles.css", "/app.js", "/icon.svg", "/manifest.webmanifest"];
+
+self.addEventListener("install", event => {
+  event.waitUntil(caches.open(CACHE).then(cache => cache.addAll(SHELL)));
+  self.skipWaiting();
+});
+
+self.addEventListener("activate", event => {
+  event.waitUntil(caches.keys().then(keys => Promise.all(keys.filter(key => key !== CACHE).map(key => caches.delete(key)))));
+  self.clients.claim();
+});
+
+self.addEventListener("fetch", event => {
+  const url = new URL(event.request.url);
+  if (event.request.method !== "GET" || url.origin !== self.location.origin || url.pathname.startsWith("/v1/") || url.pathname === "/health") return;
+  event.respondWith(fetch(event.request).then(response => {
+    const copy = response.clone();
+    caches.open(CACHE).then(cache => cache.put(event.request, copy));
+    return response;
+  }).catch(() => caches.match(event.request).then(response => response || caches.match("/"))));
+});