refactor: remove dead ultrawork model override code

Remove ultrawork-model-override hook and per-agent ultrawork model swap
config that relied on zen opencode.ai free tier (no longer functional).

Removed:
- src/hooks/ultrawork-model-override/ (hook, test, index)
- ultrawork field from AgentOverrideConfigSchema
- ultrawork-model-override from HookNameSchema
- UltraworkConfig type from model-fallback-types
- Non-max20 sonnet+ultrawork-opus codepath from model-fallback
- Claude subscription model table from installation docs
- All references in plugin-interface, create-session-hooks, schema.json
- Related test cases and updated snapshots

src/plugin-interface.ts

@@ -31,7 +31,6 @@ export function createPluginInterface(args: {
     tool: tools,
 
     "chat.params": async (input, output) => {
-      await hooks.ultraworkModelOverride?.["chat.params"]?.(input, output)
       const handler = createChatParamsHandler({ anthropicEffort: hooks.anthropicEffort })
       await handler(input, output)
     },