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/config/schema/agent-overrides.ts

@@ -32,11 +32,6 @@ export const AgentOverrideConfigSchema = z.object({
       budgetTokens: z.number().optional(),
     })
     .optional(),
-  /** Ultrawork model override configuration. */
-  ultrawork: z.object({
-    model: z.string(),
-    variant: z.string().optional(),
-  }).optional(),
   /** Reasoning effort level (OpenAI). Overrides category and default settings. */
   reasoningEffort: z.enum(["low", "medium", "high", "xhigh"]).optional(),
   /** Text verbosity level. */

src/config/schema/hooks.ts

@@ -25,7 +25,6 @@ export const HookNameSchema = z.enum([
   "interactive-bash-session",
 
   "thinking-block-validator",
-  "ultrawork-model-override",
   "ralph-loop",
   "category-skill-reminder",