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>

internal/httpapi/web.go

@@ -1,34 +0,0 @@
-package httpapi
-
-import (
-	"embed"
-	"io/fs"
-	"net/http"
-	"path"
-	"strings"
-)
-
-//go:embed web/*
-var embeddedWeb embed.FS
-
-var webContent = func() http.Handler {
-	content, err := fs.Sub(embeddedWeb, "web")
-	if err != nil {
-		panic(err)
-	}
-	return http.FileServer(http.FS(content))
-}()
-
-func (api *API) webApp(w http.ResponseWriter, r *http.Request) {
-	if strings.HasPrefix(r.URL.Path, "/v1/") {
-		writeError(w, http.StatusNotFound, "not_found", "Endpoint not found")
-		return
-	}
-	cleaned := path.Clean(r.URL.Path)
-	if cleaned == "." || cleaned == "/" || cleaned == "/index.html" || cleaned == "/service-worker.js" {
-		w.Header().Set("Cache-Control", "no-cache")
-	} else {
-		w.Header().Set("Cache-Control", "public, max-age=3600")
-	}
-	webContent.ServeHTTP(w, r)
-}