feat(agents): expose thinking variant configuration in agent settings (#1715)

* feat(agents): expose thinking variant configuration in agent settings

Fix #1425: add variant field to agent config UI so users can configure
thinking/reasoning depth per agent without editing opencode.json.

Changes:
- Added variant to AgentConfig and AgentDraft types in useAgentsStore
- Pass variant in createAgent and updateAgent API calls
- Support null for temperature, top_p, and variant to clear overrides
- Added variant input field in AgentsPage 'Model & Parameters' section
- Added variant to settings search registry
- Added i18n strings for variant field in all 8 non-English locales

The variant field maps to provider-specific parameters (e.g. Anthropic
high/max variant, OpenAI reasoning effort). Users can enter any string
value; the SDK passes it through to the model provider.

Clearing temperature/topP/variant now sends null to the server instead
of omitting the field, which properly removes the override in
opencode.json.

* fix(sync): preserve tool state.time in materialization merge

* chore: trigger re-review

* fix(agents): use thinking variant selector in settings

* fix(agents): preserve thinking variant values

---------

Co-authored-by: Leonid Skorobogatyy <bash@opencode.itc.local>

packages/ui/src/lib/settings/search.ts

@@ -428,6 +428,13 @@ export const SETTINGS_SEARCH_ITEMS: readonly SettingsSearchItem[] = [
     titleKey: 'settings.agents.page.field.overrideModel',
     keywords: ['model', 'provider'],
   },
+  {
+    id: 'agents.variant',
+    page: 'agents',
+    titleKey: 'settings.agents.page.field.variant',
+    descriptionKey: 'settings.agents.page.field.variantTooltip',
+    keywords: ['thinking', 'reasoning', 'variant', 'depth'],
+  },
   {
     id: 'agents.temperature',
     page: 'agents',