fix(agents): exclude subagents from UI model selection override

Subagents (explore, librarian, oracle, etc.) now use their own fallback
chain instead of inheriting the UI-selected model. This fixes the issue
where explore agent was incorrectly using Opus instead of Haiku.

- Add AgentMode type and static mode property to AgentFactory
- Each agent declares its own mode via factory.mode = MODE pattern
- createBuiltinAgents() checks source.mode before passing uiSelectedModel

src/agents/types.ts

@@ -1,6 +1,20 @@
 import type { AgentConfig } from "@opencode-ai/sdk"
 
-export type AgentFactory = (model: string) => AgentConfig
+/**
+ * Agent mode determines UI model selection behavior:
+ * - "primary": Respects user's UI-selected model (sisyphus, atlas)
+ * - "subagent": Uses own fallback chain, ignores UI selection (oracle, explore, etc.)
+ * - "all": Available in both contexts (OpenCode compatibility)
+ */
+export type AgentMode = "primary" | "subagent" | "all"
+
+/**
+ * Agent factory function with static mode property.
+ * Mode is exposed as static property for pre-instantiation access.
+ */
+export type AgentFactory = ((model: string) => AgentConfig) & {
+  mode: AgentMode
+}
 
 /**
  * Agent category for grouping in Sisyphus prompt sections