Flatten proxy config to a bool (drop the caddy key)

CaddySpec had only `enable` left (path_prefix/host removed; extra_snippets was
unused), so `proxy: { caddy: {} }` was three levels of nesting for a checkbox —
and it invited drift (supabase silently reverted to proxy.caddy.host). Replace it
with a plain `ServiceSpec.proxy: bool`.

- Model: remove ProxySpec/CaddySpec; `proxy: bool = False`. Readers simplified to
  `bool(svc.proxy)` (generator, castle-api summaries, CLI info).
- CLI create / dashboard editor + create form: write `proxy: true` / a checkbox.
- Configs migrated to `proxy: true` (and supabase's stale proxy.caddy.host block
  removed).
- Docs (registry, dns-and-tls, design, stack guides) + tests/fixtures updated;
  also fixed lingering path-model staleness in the stack guides.

app/src/components/detail/ServiceFields.tsx

@@ -18,7 +18,6 @@ export function ServiceFields({ service, onSave, onDelete }: Props) {
   const run = obj(m.run)
   const internal = obj(obj(obj(m.expose).http).internal)
   const httpExpose = obj(obj(m.expose).http)
-  const caddyRaw = obj(m.proxy).caddy
 
   const [saving, setSaving] = useState(false)
   const [saved, setSaved] = useState(false)
@@ -29,8 +28,8 @@ export function ServiceFields({ service, onSave, onDelete }: Props) {
   )
   const [port, setPort] = useState(internal.port != null ? String(internal.port) : "")
   const [health, setHealth] = useState((httpExpose.health_path as string) ?? "")
-  // Exposed at <service-name>.<gateway.domain> when proxy.caddy is present + enabled.
-  const [expose, setExpose] = useState(caddyRaw !== undefined && obj(caddyRaw).enable !== false)
+  // Exposed at <service-name>.<gateway.domain> when proxy is true.
+  const [expose, setExpose] = useState(m.proxy === true)
 
   const { element: envEditor, merged } = useEnvSecrets(obj(obj(m.defaults).env) as Record<string, string>)
 
@@ -62,7 +61,7 @@ export function ServiceFields({ service, onSave, onDelete }: Props) {
         delete config.expose
       }
 
-      if (expose) config.proxy = { caddy: {} }
+      if (expose) config.proxy = true
       else delete config.proxy
 
       const env = merged()