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.ts

@@ -0,0 +1,34 @@
+export type SessionPromptParams = {
+  temperature?: number
+  topP?: number
+  options?: Record<string, unknown>
+}
+
+const sessionPromptParams = new Map<string, SessionPromptParams>()
+
+export function setSessionPromptParams(sessionID: string, params: SessionPromptParams): void {
+  sessionPromptParams.set(sessionID, {
+    ...(params.temperature !== undefined ? { temperature: params.temperature } : {}),
+    ...(params.topP !== undefined ? { topP: params.topP } : {}),
+    ...(params.options !== undefined ? { options: { ...params.options } } : {}),
+  })
+}
+
+export function getSessionPromptParams(sessionID: string): SessionPromptParams | undefined {
+  const params = sessionPromptParams.get(sessionID)
+  if (!params) return undefined
+
+  return {
+    ...(params.temperature !== undefined ? { temperature: params.temperature } : {}),
+    ...(params.topP !== undefined ? { topP: params.topP } : {}),
+    ...(params.options !== undefined ? { options: { ...params.options } } : {}),
+  }
+}
+
+export function clearSessionPromptParams(sessionID: string): void {
+  sessionPromptParams.delete(sessionID)
+}
+
+export function clearAllSessionPromptParams(): void {
+  sessionPromptParams.clear()
+}