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/shared/session-prompt-params-state.test.ts

@@ -0,0 +1,65 @@
+import { afterEach, describe, expect, test } from "bun:test"
+
+import {
+  clearAllSessionPromptParams,
+  clearSessionPromptParams,
+  getSessionPromptParams,
+  setSessionPromptParams,
+} from "./session-prompt-params-state"
+
+describe("session-prompt-params-state", () => {
+  afterEach(() => {
+    clearAllSessionPromptParams()
+  })
+
+  test("stores and returns prompt params by session", () => {
+    //#given
+    const sessionID = "ses_prompt_params"
+    const params = {
+      temperature: 0.4,
+      topP: 0.7,
+      options: {
+        reasoningEffort: "high",
+        maxTokens: 4096,
+      },
+    }
+
+    //#when
+    setSessionPromptParams(sessionID, params)
+
+    //#then
+    expect(getSessionPromptParams(sessionID)).toEqual(params)
+  })
+
+  test("returns copies so callers cannot mutate stored state", () => {
+    //#given
+    const sessionID = "ses_prompt_params_copy"
+    setSessionPromptParams(sessionID, {
+      temperature: 0.2,
+      options: { reasoningEffort: "medium" },
+    })
+
+    //#when
+    const result = getSessionPromptParams(sessionID)!
+    result.temperature = 0.9
+    result.options!.reasoningEffort = "max"
+
+    //#then
+    expect(getSessionPromptParams(sessionID)).toEqual({
+      temperature: 0.2,
+      options: { reasoningEffort: "medium" },
+    })
+  })
+
+  test("clears a single session", () => {
+    //#given
+    const sessionID = "ses_prompt_params_clear"
+    setSessionPromptParams(sessionID, { topP: 0.5 })
+
+    //#when
+    clearSessionPromptParams(sessionID)
+
+    //#then
+    expect(getSessionPromptParams(sessionID)).toBeUndefined()
+  })
+})