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/plugin/chat-params.test.ts

@@ -1,8 +1,17 @@
-import { describe, expect, test } from "bun:test"
+import { afterEach, describe, expect, test } from "bun:test"
 
 import { createChatParamsHandler } from "./chat-params"
+import {
+  clearSessionPromptParams,
+  getSessionPromptParams,
+  setSessionPromptParams,
+} from "../shared/session-prompt-params-state"
 
 describe("createChatParamsHandler", () => {
+  afterEach(() => {
+    clearSessionPromptParams("ses_chat_params")
+  })
+
   test("normalizes object-style agent payload and runs chat.params hooks", async () => {
     //#given
     let called = false
@@ -35,7 +44,6 @@ describe("createChatParamsHandler", () => {
     //#then
     expect(called).toBe(true)
   })
-
   test("passes the original mutable message object to chat.params hooks", async () => {
     //#given
     const handler = createChatParamsHandler({
@@ -68,4 +76,61 @@ describe("createChatParamsHandler", () => {
     //#then
     expect(message.variant).toBe("high")
   })
+
+  test("applies stored prompt params for the session", async () => {
+    //#given
+    setSessionPromptParams("ses_chat_params", {
+      temperature: 0.4,
+      topP: 0.7,
+      options: {
+        reasoningEffort: "high",
+        thinking: { type: "disabled" },
+        maxTokens: 4096,
+      },
+    })
+
+    const handler = createChatParamsHandler({
+      anthropicEffort: null,
+    })
+
+    const input = {
+      sessionID: "ses_chat_params",
+      agent: { name: "oracle" },
+      model: { providerID: "openai", modelID: "gpt-5.4" },
+      provider: { id: "openai" },
+      message: {},
+    }
+
+    const output = {
+      temperature: 0.1,
+      topP: 1,
+      topK: 1,
+      options: { existing: true },
+    }
+
+    //#when
+    await handler(input, output)
+
+    //#then
+    expect(output).toEqual({
+      temperature: 0.4,
+      topP: 0.7,
+      topK: 1,
+      options: {
+        existing: true,
+        reasoningEffort: "high",
+        thinking: { type: "disabled" },
+        maxTokens: 4096,
+      },
+    })
+    expect(getSessionPromptParams("ses_chat_params")).toEqual({
+      temperature: 0.4,
+      topP: 0.7,
+      options: {
+        reasoningEffort: "high",
+        thinking: { type: "disabled" },
+        maxTokens: 4096,
+      },
+    })
+  })
 })