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/hooks/use-agent-initialization.ts

@@ -60,7 +60,7 @@ export function useAgentInitialization({
       const hasAuthoritativeHistory =
         session?.agentAuthoritativeHistoryApplied.get(agentId) === true;
       const timelineRequest = deriveInitialTimelineRequest({
-        cursor: cursor ? { seq: cursor.endSeq } : null,
+        cursor: cursor ? { epoch: cursor.epoch, seq: cursor.endSeq } : null,
         hasAuthoritativeHistory,
         initialTimelineLimit,
       });
@@ -107,6 +107,7 @@ export function useAgentInitialization({
         await client.fetchAgentTimeline(agentId, {
           direction: "tail",
           limit: initialTimelineLimit,
+          projection: "canonical",
         });
       } catch (error) {
         setAgentInitializing(agentId, false);