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.ts

@@ -76,15 +76,6 @@ export function generateModelConfig(config: InstallConfig): GeneratedOmoConfig {
         continue
       }
 
-      if (avail.native.claude && !avail.isMaxPlan) {
-        agents[role] = {
-          model: "anthropic/claude-sonnet-4-6",
-          variant: "max",
-          ultrawork: { model: "anthropic/claude-opus-4-6", variant: "max" },
-        }
-        continue
-      }
-
       const resolved = resolveModelFromChain(fallbackChain, avail)
       if (resolved) {
         const variant = resolved.variant ?? req.variant