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/__snapshots__/model-fallback.test.ts.snap

@@ -94,11 +94,7 @@ exports[`generateModelConfig single native provider uses Claude models when only
       "variant": "max",
     },
     "sisyphus": {
-      "model": "anthropic/claude-sonnet-4-6",
-      "ultrawork": {
-        "model": "anthropic/claude-opus-4-6",
-        "variant": "max",
-      },
+      "model": "anthropic/claude-opus-4-6",
       "variant": "max",
     },
   },
@@ -483,11 +479,7 @@ exports[`generateModelConfig all native providers uses preferred models from fal
       "variant": "max",
     },
     "sisyphus": {
-      "model": "anthropic/claude-sonnet-4-6",
-      "ultrawork": {
-        "model": "anthropic/claude-opus-4-6",
-        "variant": "max",
-      },
+      "model": "anthropic/claude-opus-4-6",
       "variant": "max",
     },
   },
@@ -1044,11 +1036,7 @@ exports[`generateModelConfig mixed provider scenarios uses Claude + OpenCode Zen
       "variant": "max",
     },
     "sisyphus": {
-      "model": "anthropic/claude-sonnet-4-6",
-      "ultrawork": {
-        "model": "anthropic/claude-opus-4-6",
-        "variant": "max",
-      },
+      "model": "anthropic/claude-opus-4-6",
       "variant": "max",
     },
   },
@@ -1192,11 +1180,7 @@ exports[`generateModelConfig mixed provider scenarios uses Claude + ZAI combinat
       "variant": "max",
     },
     "sisyphus": {
-      "model": "anthropic/claude-sonnet-4-6",
-      "ultrawork": {
-        "model": "anthropic/claude-opus-4-6",
-        "variant": "max",
-      },
+      "model": "anthropic/claude-opus-4-6",
       "variant": "max",
     },
   },
@@ -1257,11 +1241,7 @@ exports[`generateModelConfig mixed provider scenarios uses Gemini + Claude combi
       "variant": "max",
     },
     "sisyphus": {
-      "model": "anthropic/claude-sonnet-4-6",
-      "ultrawork": {
-        "model": "anthropic/claude-opus-4-6",
-        "variant": "max",
-      },
+      "model": "anthropic/claude-opus-4-6",
       "variant": "max",
     },
   },
@@ -1405,11 +1385,7 @@ exports[`generateModelConfig mixed provider scenarios uses all providers togethe
       "variant": "max",
     },
     "sisyphus": {
-      "model": "anthropic/claude-sonnet-4-6",
-      "ultrawork": {
-        "model": "anthropic/claude-opus-4-6",
-        "variant": "max",
-      },
+      "model": "anthropic/claude-opus-4-6",
       "variant": "max",
     },
   },