feat: model taxonomy gains gpt-5.6-sol + per-host generation defaults

Adds 'gpt-5.6-sol' to the model taxonomy with exact-match-only resolution
(Terra/Luna/suffixed IDs deliberately fall back to generic gpt) and replaces
the hardcoded 'claude' generation default with a validated
HostConfig.defaultModel: codex renders the gpt profile when --model is
absent, every other host keeps claude. Codex ship golden regenerated
accordingly; ADDING_A_HOST documents the new field.

scripts/host-config.ts

@@ -14,6 +14,9 @@
  *                                              platform-detect, uninstall
  */
 
+import type { Model } from './models';
+import { validateModel } from './models';
+
 export interface HostConfig {
   /** Unique host identifier (e.g., 'opencode'). Must match filename in hosts/. */
   name: string;
@@ -24,6 +27,9 @@ export interface HostConfig {
   /** Alternative binary names (e.g., ['droid'] for factory). */
   cliAliases?: string[];
 
+  /** Model overlay used when generation does not receive an explicit --model. */
+  defaultModel: Model;
+
   // --- Path Configuration ---
   /** Global install path relative to $HOME (e.g., '.config/opencode/skills/gstack'). */
   globalRoot: string;
@@ -119,6 +125,10 @@ export function validateHostConfig(config: HostConfig, validResolverNames?: Read
       }
     }
   }
+  const modelError = validateModel(config.defaultModel);
+  if (modelError) {
+    errors.push(`defaultModel ${modelError}`);
+  }
   if (!PATH_REGEX.test(config.globalRoot)) {
     errors.push(`globalRoot '${config.globalRoot}' contains invalid characters`);
   }