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/conftest.py

@@ -58,9 +58,7 @@ def castle_root(tmp_path: Path) -> Generator[Path, None, None]:
                         "health_path": "/health",
                     }
                 },
-                "proxy": {
-                    "caddy": {"path_prefix": "/test-svc"},
-                },
+                "proxy": True,
                 "manage": {
                     "systemd": {},
                 },