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.

core/tests/test_config.py

@@ -46,7 +46,7 @@ class TestLoadConfig:
         """Service has correct proxy spec."""
         config = load_config(castle_root)
         svc = config.services["test-svc"]
-        assert svc.proxy.caddy.enable is True  # exposed at <name>.<gateway.domain>
+        assert svc.proxy is True  # exposed at <name>.<gateway.domain>
 
     def test_service_run_spec(self, castle_root: Path) -> None:
         """Service has correct RunSpec."""