Subdomain-only gateway routing; drop path prefixes

Collapse the two proxy shapes (path_prefix + host) to a single checkbox: a
service is either port-only, or exposed at <service-name>.<gateway.domain>. Path
routes and their prefix-stripping foot-guns are gone; the subdomain is always the
service name (rename the service to change it).

- Schema: CaddySpec is just `enable` (presence = expose). service_proxy_targets
  returns (expose, port, base_url); Deployment carries `subdomain` (was
  proxy_path/proxy_host). Registry/deploy/mesh/CLI updated in lockstep.
- Generator: compute_routes emits one host route per exposed service/frontend
  (address = name). acme mode = one *.<domain> site with a reverse_proxy matcher
  per service + a file_server matcher per frontend; the :<port> site redirects to
  the dashboard. off mode = a HTTP control plane on :<port> (dashboard at / +
  /api → castle-api); other services are port-only.
- Dashboard/API: castle-app and castle-api are ordinary subdomains. The dashboard
  derives the API base at runtime (castle-api.<domain> on a subdomain, else /api)
  and calls it cross-origin — castle-api already allows CORS *. Frontends build
  with VITE_BASE=/ (served at their subdomain root).
- CLI: `service create` drops --path/--host; --no-proxy makes it port-only.
- Docs/tests reworked for the model; docs use generic placeholders only (no
  personal host/domain/IP details).

core/src/castle_core/registry.py

@@ -52,8 +52,9 @@ class Deployment:
     stack: str | None = None
     port: int | None = None
     health_path: str | None = None
-    proxy_path: str | None = None
-    proxy_host: str | None = None
+    # Exposed at <subdomain>.<gateway.domain> (the subdomain is the service name),
+    # or None when the service is reachable only at its host:port.
+    subdomain: str | None = None
     base_url: str | None = None
     schedule: str | None = None
     managed: bool = False
@@ -121,8 +122,7 @@ def load_registry(path: Path | None = None) -> NodeRegistry:
             stack=comp_data.get("stack"),
             port=comp_data.get("port"),
             health_path=comp_data.get("health_path"),
-            proxy_path=comp_data.get("proxy_path"),
-            proxy_host=comp_data.get("proxy_host"),
+            subdomain=comp_data.get("subdomain"),
             base_url=comp_data.get("base_url"),
             schedule=comp_data.get("schedule"),
             managed=comp_data.get("managed", False),
@@ -178,10 +178,8 @@ def save_registry(registry: NodeRegistry, path: Path | None = None) -> None:
             entry["port"] = comp.port
         if comp.health_path:
             entry["health_path"] = comp.health_path
-        if comp.proxy_path:
-            entry["proxy_path"] = comp.proxy_path
-        if comp.proxy_host:
-            entry["proxy_host"] = comp.proxy_host
+        if comp.subdomain:
+            entry["subdomain"] = comp.subdomain
         if comp.base_url:
             entry["base_url"] = comp.base_url
         if comp.schedule: