feat(agent-model): add per-agent model override with default-reset UX and runtime sync (#651)

src/types/agent.ts

@@ -3,6 +3,8 @@ export interface AgentSummary {
   name: string;
   isDefault: boolean;
   modelDisplay: string;
+  modelRef?: string | null;
+  overrideModelRef?: string | null;
   inheritedModel: boolean;
   workspace: string;
   agentDir: string;
@@ -13,6 +15,7 @@ export interface AgentSummary {
 export interface AgentsSnapshot {
   agents: AgentSummary[];
   defaultAgentId: string;
+  defaultModelRef?: string | null;
   configuredChannelTypes: string[];
   channelOwners: Record<string, string>;
   channelAccountOwners: Record<string, string>;