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/known-variants.ts

@@ -0,0 +1,16 @@
+/**
+ * Canonical set of recognised variant / effort tokens.
+ * Used by parseFallbackModelEntry (space-suffix detection) and
+ * flattenToFallbackModelStrings (inline-variant stripping).
+ */
+export const KNOWN_VARIANTS = new Set([
+  "low",
+  "medium",
+  "high",
+  "xhigh",
+  "max",
+  "minimal",
+  "none",
+  "auto",
+  "thinking",
+])