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/contexts/session-timeline-seq-gate.ts

@@ -1,22 +1,28 @@
 export type SessionTimelineSeqCursor =
   | {
+      epoch: string;
       endSeq: number;
     }
   | null
   | undefined;
 
-export type SessionTimelineSeqDecision = "accept" | "drop_stale" | "gap" | "init";
+export type SessionTimelineSeqDecision = "accept" | "drop_stale" | "drop_epoch" | "gap" | "init";
 
 export function classifySessionTimelineSeq({
   cursor,
+  epoch,
   seq,
 }: {
   cursor: SessionTimelineSeqCursor;
+  epoch: string;
   seq: number;
 }): SessionTimelineSeqDecision {
   if (!cursor) {
     return "init";
   }
+  if (cursor.epoch !== epoch) {
+    return "drop_epoch";
+  }
   if (seq <= cursor.endSeq) {
     return "drop_stale";
   }