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/cli/model-fallback-types.ts

@@ -11,15 +11,9 @@ export interface ProviderAvailability {
 	isMaxPlan: boolean
 }
 
-export interface UltraworkConfig {
-	model: string
-	variant?: string
-}
-
 export interface AgentConfig {
 	model: string
 	variant?: string
-	ultrawork?: UltraworkConfig
 }
 
 export interface CategoryConfig {