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/i18n/messages/uk.settings.ts

@@ -387,6 +387,10 @@ export const settingsDict = {
   "settings.agents.page.field.mode": "Режим",
   "settings.agents.page.field.modeTooltip": "Видимість основного або субагента",
   "settings.agents.page.field.overrideModel": "Перевизначити модель",
+  "settings.agents.page.field.variant": "Варіант мислення",
+  "settings.agents.page.field.variantTooltip": "Необов'язково. Якщо модель підтримує міркування, варіант контролює глибину міркування.",
+  "settings.agents.page.field.variantHint": "Специфічно для моделі (напр. варіант Anthropic)",
+  "settings.agents.page.field.variantPlaceholder": "напр. high, max, low, none",
   "settings.agents.page.field.temperature": "Температура",
   "settings.agents.page.field.temperatureTooltip": "Контролює випадковість. Вищий = творчий, нижчий = зосереджений.",
   "settings.agents.page.field.temperatureRange": "0,0 до 2,0",