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.

hosts/codex.ts

@@ -4,6 +4,7 @@ const codex = defineHost({
   name: 'codex',
   displayName: 'OpenAI Codex CLI',
   cliAliases: ['agents'],
+  defaultModel: 'gpt',
 
   localSkillRoot: '.agents/skills/gstack',
   hostSubdir: '.agents',