fix(app): paginate canonical timeline catch-up to avoid relay 1009 disconnect loop (#657)

@fireblue diagnosed the production failure mode: long canonical timeline catch-up could exceed the relay WebSocket frame cap, close with 1009, and send clients into a reconnect loop. Their seatbelt fix also removed the `limit: 0` footgun that let app callers request an unbounded frame.

This maintainer pass keeps that diagnosis and redirects the UX around bounded projected pages:

- initial agent load fetches the latest canonical tail
- app resume/reconnect re-fetches the latest tail instead of chaining after-cursor pages
- in-stream gaps still use bounded after-cursor catch-up
- older history loads explicitly when the user scrolls to the oldest edge
- shared page size is TIMELINE_FETCH_PAGE_SIZE = 100 projected timeline items, matching the daemon's canonical projection semantics rather than raw delta chunks

Original diagnosis and fix by @fireblue.

Co-authored-by: Mohamed Boudra <boudra.moha@gmail.com>

packages/app/src/contexts/session-context.tsx

@@ -12,7 +12,8 @@ import {
   processTimelineResponse,
   type ProcessTimelineResponseOutput,
   type TimelineReducerSideEffect,
-} from "@/contexts/session-stream-reducers";
+} from "@/timeline/session-stream-reducers";
+import { TIMELINE_FETCH_PAGE_SIZE } from "@/timeline/timeline-fetch-policy";
 import type {
   AgentAttachment,
   AgentStreamEventPayload,
@@ -451,6 +452,7 @@ function SessionProviderInternal({ children, serverId, client }: SessionProvider
   const setAgentStreamState = useSessionStore((state) => state.setAgentStreamState);
   const clearAgentStreamHead = useSessionStore((state) => state.clearAgentStreamHead);
   const setAgentTimelineCursor = useSessionStore((state) => state.setAgentTimelineCursor);
+  const setAgentTimelineHasOlder = useSessionStore((state) => state.setAgentTimelineHasOlder);
   const setInitializingAgents = useSessionStore((state) => state.setInitializingAgents);
   const bumpHistorySyncGeneration = useSessionStore((state) => state.bumpHistorySyncGeneration);
   const markAgentHistorySynchronized = useSessionStore(
@@ -748,17 +750,15 @@ function SessionProviderInternal({ children, serverId, client }: SessionProvider
       if (isNative) {
         const session = useSessionStore.getState().sessions[serverId];
         const agentId = session?.focusedAgentId;
-        const cursor = agentId ? session?.agentTimelineCursor.get(agentId) : undefined;
-        if (agentId && cursor) {
+        if (agentId) {
           void client
             .fetchAgentTimeline(agentId, {
-              direction: "after",
-              cursor: { epoch: cursor.epoch, seq: cursor.endSeq },
-              limit: 0,
+              direction: "tail",
+              limit: TIMELINE_FETCH_PAGE_SIZE,
               projection: "canonical",
             })
             .catch((error) => {
-              console.warn("[Session] failed to fetch catch-up timeline on resume", agentId, error);
+              console.warn("[Session] failed to fetch tail timeline on resume", agentId, error);
             });
         }
       }
@@ -1024,7 +1024,7 @@ function SessionProviderInternal({ children, serverId, client }: SessionProvider
         .fetchAgentTimeline(agentId, {
           direction: "after",
           cursor: { epoch: cursor.epoch, seq: cursor.endSeq },
-          limit: 0,
+          limit: TIMELINE_FETCH_PAGE_SIZE,
           projection: "canonical",
         })
         .catch((error) => {
@@ -1055,6 +1055,15 @@ function SessionProviderInternal({ children, serverId, client }: SessionProvider
       const currentTail = session?.agentStreamTail.get(agentId) ?? [];
       const currentHead = session?.agentStreamHead.get(agentId) ?? [];
 
+      setAgentTimelineHasOlder(serverId, (prev) => {
+        if (prev.get(agentId) === payload.hasOlder) {
+          return prev;
+        }
+        const next = new Map(prev);
+        next.set(agentId, payload.hasOlder);
+        return next;
+      });
+
       if (payload.agent) {
         const normalized = normalizeAgentSnapshot(payload.agent, serverId);
         applyAuthoritativeAgentSnapshot({
@@ -1127,6 +1136,7 @@ function SessionProviderInternal({ children, serverId, client }: SessionProvider
       setAgentStreamHead,
       setAgentStreamTail,
       setAgentTimelineCursor,
+      setAgentTimelineHasOlder,
       setInitializingAgents,
     ],
   );