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_manifest.py

@@ -5,14 +5,12 @@ from __future__ import annotations
 import pytest
 from castle_core.manifest import (
     BuildSpec,
-    CaddySpec,
     ProgramSpec,
     ExposeSpec,
     HttpExposeSpec,
     HttpInternal,
     JobSpec,
     ManageSpec,
-    ProxySpec,
     RunCommand,
     RunPython,
     RunRemote,
@@ -92,9 +90,9 @@ class TestServiceSpec:
         s = ServiceSpec(
             id="svc",
             run=RunPython(runner="python", program="svc"),
-            proxy=ProxySpec(caddy=CaddySpec()),
+            proxy=True,
         )
-        assert s.proxy.caddy.enable is True
+        assert s.proxy is True
 
     def test_service_with_manage(self) -> None:
         """Service with systemd management."""
@@ -185,10 +183,10 @@ class TestModelSerialization:
                     internal=HttpInternal(port=9001), health_path="/health"
                 )
             ),
-            proxy=ProxySpec(caddy=CaddySpec()),
+            proxy=True,
             manage=ManageSpec(systemd=SystemdSpec()),
         )
         data = s.model_dump(exclude_none=True, exclude={"id"})
         assert data["run"]["runner"] == "python"
         assert data["expose"]["http"]["internal"]["port"] == 9001
-        assert data["proxy"]["caddy"]["enable"] is True
+        assert data["proxy"] is True