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/model-resolver.ts

@@ -1,6 +1,8 @@
 import type { FallbackEntry } from "./model-requirements"
+import type { FallbackModelObject } from "../config/schema/fallback-models"
 import { normalizeModel } from "./model-normalization"
 import { resolveModelPipeline } from "./model-resolution-pipeline"
+import { KNOWN_VARIANTS } from "./known-variants"
 
 export type ModelResolutionInput = {
 	userModel?: string
@@ -61,11 +63,45 @@ export function resolveModelWithFallback(
 }
 
 /**
- * Normalizes fallback_models config (which can be string or string[]) to string[]
- * Centralized helper to avoid duplicated normalization logic
+ * Normalizes fallback_models config to a mixed array.
+ * Accepts string, string[], or mixed arrays of strings and FallbackModelObject entries.
  */
-export function normalizeFallbackModels(models: string | string[] | undefined): string[] | undefined {
+export function normalizeFallbackModels(
+	models: string | (string | FallbackModelObject)[] | undefined,
+): (string | FallbackModelObject)[] | undefined {
 	if (!models) return undefined
 	if (typeof models === "string") return [models]
 	return models
 }
+
+/**
+ * Extracts plain model strings from a mixed fallback models array.
+ * Object entries are flattened to "model" or "model(variant)" strings.
+ * Use this when consumers need string[] (e.g., resolveModelForDelegateTask).
+ */
+export function flattenToFallbackModelStrings(
+	models: (string | FallbackModelObject)[] | undefined,
+): string[] | undefined {
+	if (!models) return undefined
+	return models.map((entry) => {
+		if (typeof entry === "string") return entry
+		const variant = entry.variant
+		if (variant) {
+			// Strip any supported inline variant syntax before appending explicit override.
+			// Supports both parenthesized and space-suffix forms so we don't emit
+			// invalid strings like "provider/model high(low)".
+			const model = entry.model
+				.replace(/\([^()]+\)\s*$/, "")
+				.replace(/\s+([a-z][a-z0-9_-]*)\s*$/i, (match, suffix) => {
+					const normalized = String(suffix).toLowerCase()
+					return KNOWN_VARIANTS.has(normalized)
+						? ""
+						: match
+				})
+				.trim()
+			return `${model}(${variant})`
+		}
+		// No explicit variant — preserve model string as-is (including any inline variant)
+		return entry.model
+	})
+}