feat(config): support object-style fallback_models with per-model settings

Add support for object-style entries in fallback_models arrays, enabling
per-model configuration of variant, reasoningEffort, temperature, top_p,
maxTokens, and thinking settings.

- Zod schema for FallbackModelObject with full validation
- normalizeFallbackModels() and flattenToFallbackModelStrings() utilities
- Provider-agnostic model resolution pipeline with fallback chain
- Session prompt params state management
- Fallback chain construction with prefix-match lookup
- Integration across delegate-task, background-agent, and plugin layers

src/features/background-agent/spawner.ts

@@ -2,6 +2,7 @@ import type { BackgroundTask, LaunchInput, ResumeInput } from "./types"
 import type { OpencodeClient, OnSubagentSessionCreated, QueueItem } from "./constants"
 import { TMUX_CALLBACK_DELAY_MS } from "./constants"
 import { log, getAgentToolRestrictions, promptWithModelSuggestionRetry, createInternalAgentTextPart } from "../../shared"
+import { setSessionPromptParams } from "../../shared/session-prompt-params-state"
 import { subagentSessions } from "../claude-code-session-state"
 import { getTaskToastManager } from "../task-toast-manager"
 import { isInsideTmux } from "../../shared/tmux"
@@ -128,10 +129,33 @@ export async function startTask(
   })
 
   const launchModel = input.model
-    ? { providerID: input.model.providerID, modelID: input.model.modelID }
+    ? {
+        providerID: input.model.providerID,
+        modelID: input.model.modelID,
+      }
     : undefined
   const launchVariant = input.model?.variant
 
+  if (input.model) {
+    const promptOptions: Record<string, unknown> = {
+      ...(input.model.reasoningEffort ? { reasoningEffort: input.model.reasoningEffort } : {}),
+      ...(input.model.thinking ? { thinking: input.model.thinking } : {}),
+      ...(input.model.maxTokens !== undefined ? { maxTokens: input.model.maxTokens } : {}),
+    }
+
+    if (
+      input.model.temperature !== undefined ||
+      input.model.top_p !== undefined ||
+      Object.keys(promptOptions).length > 0
+    ) {
+      setSessionPromptParams(sessionID, {
+        ...(input.model.temperature !== undefined ? { temperature: input.model.temperature } : {}),
+        ...(input.model.top_p !== undefined ? { topP: input.model.top_p } : {}),
+        ...(Object.keys(promptOptions).length > 0 ? { options: promptOptions } : {}),
+      })
+    }
+  }
+
   promptWithModelSuggestionRetry(client, {
     path: { id: sessionID },
     body: {
@@ -213,10 +237,33 @@ export async function resumeTask(
   })
 
   const resumeModel = task.model
-    ? { providerID: task.model.providerID, modelID: task.model.modelID }
+    ? {
+        providerID: task.model.providerID,
+        modelID: task.model.modelID,
+      }
     : undefined
   const resumeVariant = task.model?.variant
 
+  if (task.model) {
+    const promptOptions: Record<string, unknown> = {
+      ...(task.model.reasoningEffort ? { reasoningEffort: task.model.reasoningEffort } : {}),
+      ...(task.model.thinking ? { thinking: task.model.thinking } : {}),
+      ...(task.model.maxTokens !== undefined ? { maxTokens: task.model.maxTokens } : {}),
+    }
+
+    if (
+      task.model.temperature !== undefined ||
+      task.model.top_p !== undefined ||
+      Object.keys(promptOptions).length > 0
+    ) {
+      setSessionPromptParams(task.sessionID, {
+        ...(task.model.temperature !== undefined ? { temperature: task.model.temperature } : {}),
+        ...(task.model.top_p !== undefined ? { topP: task.model.top_p } : {}),
+        ...(Object.keys(promptOptions).length > 0 ? { options: promptOptions } : {}),
+      })
+    }
+  }
+
   client.session.promptAsync({
     path: { id: task.sessionID },
     body: {