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/event-compaction-agent.test.ts

@@ -2,6 +2,7 @@ import { afterEach, describe, expect, it } from "bun:test"
 
 import { _resetForTesting, getSessionAgent, updateSessionAgent } from "../features/claude-code-session-state"
 import { clearSessionModel, getSessionModel, setSessionModel } from "../shared/session-model-state"
+import { clearSessionPromptParams } from "../shared/session-prompt-params-state"
 import { createEventHandler } from "./event"
 
 function createMinimalEventHandler() {
@@ -53,6 +54,8 @@ describe("createEventHandler compaction agent filtering", () => {
     _resetForTesting()
     clearSessionModel("ses_compaction_poisoning")
     clearSessionModel("ses_compaction_model_poisoning")
+    clearSessionPromptParams("ses_compaction_poisoning")
+    clearSessionPromptParams("ses_compaction_model_poisoning")
   })
 
   it("does not overwrite the stored session agent with compaction", async () => {