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.

cli/src/castle_cli/manifest.py

@@ -3,7 +3,6 @@
 from castle_core.manifest import *  # noqa: F401, F403
 from castle_core.manifest import (  # noqa: F401 — explicit re-exports for type checkers
     BuildSpec,
-    CaddySpec,
     Capability,
     CommandsSpec,
     DefaultsSpec,
@@ -15,7 +14,6 @@ from castle_core.manifest import (  # noqa: F401 — explicit re-exports for typ
     JobSpec,
     ManageSpec,
     ProgramSpec,
-    ProxySpec,
     ReadinessHttpGet,
     RestartPolicy,
     RunBase,