Fix proxy:bool refactor drift in CLI fixture + dead structural key

The proxy->bool refactor (770574f) missed the shared CLI test fixture, which
still used proxy: {caddy: {path_prefix: ...}} — failing ~25 CLI tests with
'Input should be a valid boolean'. Set it to proxy: True.

Also drop the now-dead 'caddy' entry from _STRUCTURAL_KEYS (no nested caddy
key exists anymore) and refresh a stale proxy.caddy docstring reference.

cli/tests/conftest.py

@@ -55,9 +55,7 @@ def castle_root(tmp_path: Path) -> Generator[Path, None, None]:
                         "health_path": "/health",
                     }
                 },
-                "proxy": {
-                    "caddy": {"path_prefix": "/test-svc"},
-                },
+                "proxy": True,
                 "manage": {
                     "systemd": {},
                 },