feat(plugins): add modelOverride/providerOverride to before_agent_start hook

Enable plugins to override the model and provider for agent runs by
returning modelOverride/providerOverride from the before_agent_start
hook. The hook is now invoked early in run.ts (before resolveModel)
so overrides take effect. The result is passed to attempt.ts via
earlyHookResult to prevent double-firing.

This enables security-critical use cases like routing PII-containing
prompts to local models instead of cloud providers.

src/plugins/types.ts

@@ -332,6 +332,10 @@ export type PluginHookBeforeAgentStartEvent = {
 export type PluginHookBeforeAgentStartResult = {
   systemPrompt?: string;
   prependContext?: string;
+  /** Override the model for this agent run. E.g. "llama3.3:8b" */
+  modelOverride?: string;
+  /** Override the provider for this agent run. E.g. "ollama" */
+  providerOverride?: string;
 };
 
 // llm_input hook