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/zh-TW.settings.ts

@@ -384,6 +384,10 @@
   'settings.agents.page.field.mode': '模式',
   'settings.agents.page.field.modeTooltip': 'Primary 與 Subagent 可見性',
   'settings.agents.page.field.overrideModel': '覆寫模型',
+  'settings.agents.page.field.variant': '思考變體',
+  'settings.agents.page.field.variantTooltip': '選填。如果模型支援推理，變體控制推理深度。',
+  'settings.agents.page.field.variantHint': '模型特定（如 Anthropic variant）',
+  'settings.agents.page.field.variantPlaceholder': '例如 high, max, low, none',
   'settings.agents.page.field.temperature': 'Temperature',
   'settings.agents.page.field.temperatureTooltip': '控制隨機性。越高越有創造力，越低越聚焦。',
   'settings.agents.page.field.temperatureRange': '0.0 到 2.0',