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

docs/guide/installation.md

@@ -68,15 +68,6 @@ Ask the user these questions to determine CLI options:
 
 **Provider Priority**: Native (anthropic/, openai/, google/) > GitHub Copilot > OpenCode Zen > Z.ai Coding Plan
 
-#### Claude Subscription Model Assignments
-
-| Subscription | Sisyphus (Daily) | Ultrawork Mode |
-| ------------ | ---------------- | -------------- |
-| **max20** | `anthropic/claude-opus-4-6` (max) | Already on Opus — no override |
-| **standard** | `anthropic/claude-sonnet-4-6` (max) | `anthropic/claude-opus-4-6` (max) |
-
-Standard Claude subscribers use Sonnet 4.6 for daily driving and automatically switch to Opus 4.6 when ultrawork mode is activated (by typing `ultrawork` or `ulw`).
-
 MUST STRONGLY WARNING, WHEN USER SAID THEY DON'T HAVE CLAUDE SUBSCRIPTION, SISYPHUS AGENT MIGHT NOT WORK IDEALLY.
 
 ### Step 1: Install OpenCode (if not installed)