fix: sync dev branch with main timeline/schema changes and remove stale providers

Forward-port main's timeline epoch tracking, projected window selection,
and schema updates into dev. Remove aider/amp/gemini providers and
provider-history-compatibility layers that are no longer needed.

packages/app/src/stores/session-store.ts

@@ -212,6 +212,7 @@ export type DaemonServerInfo = {
 };
 
 export interface AgentTimelineCursorState {
+  epoch: string;
   startSeq: number;
   endSeq: number;
 }