Fix duplicated and out-of-order agent chat messages (#2185)

* fix(app): keep agent timelines ordered during catch-up

Projected catch-up pages can overlap live rows and arrive after optimistic
prompts. Reconcile canonical projections before the existing stream reducer,
and retain selective subscriptions briefly across quick view switches.

* refactor(app): make optimistic catch-up consumption explicit

* fix(app): preserve catch-up message order

* fix(app): isolate delayed catch-up history

* fix(app): anchor catch-up before live turns

* fix(app): stage complete catch-up history

* fix(app): keep timeline arrays Hermes-safe

* refactor(app): preserve prompt positions during catch-up

docs/timeline-sync.md

@@ -51,9 +51,39 @@ When a client resumes with a known cursor, it catches up after that cursor to co
 
 When a client resumes without a cursor, it fetches the latest tail page.
 
+## Selective and legacy delivery
+
+The app chooses one delivery policy from `server_info.features.selectiveAgentTimeline`:
+
+- Selective daemons receive the union of agents visible in every pane. Additions subscribe and
+  catch up immediately. Removals stay subscribed for a short grace period so quick tab and pane
+  switches do not repeatedly unsubscribe and catch up. Backgrounding, disconnecting, and disposal
+  clear that grace immediately.
+- Legacy daemons keep globally streaming agent timelines. Visibility still triggers the existing
+  authoritative catch-up, but the app does not issue selective-subscription RPCs.
+
+This policy is owned by `viewed-timeline-sync.ts`; downstream reducers do not branch on daemon
+version.
+
+## Projected pages reconcile with live presentation
+
+A projected page is canonical state, not a sequence of live deltas. One projected item can overlap
+rows already received live—for example, a tool call retained at its original display position while
+its completion advances `seqEnd`, followed by a merged assistant message. The app uses
+`sourceSeqRanges` to replace overlapping assistant and reasoning projections before applying the
+remaining page through the existing stream reducer. It must not append full projected text to a
+live prefix.
+
+Optimistic user prompts are presentation state rather than canonical history. Incremental catch-up
+temporarily separates them, applies canonical entries, lets canonical user rows reconcile through
+the existing optimistic-message rules, then restores any unmatched prompts after the caught-up
+history. This keeps late history before a newly submitted prompt without duplicating an
+acknowledged prompt.
+
 ## Relevant code
 
 - Server live stream forwarding: `packages/server/src/server/session.ts`
 - App sync planning: `packages/app/src/timeline/timeline-sync-plan.ts`
+- App viewed-agent synchronization: `packages/app/src/timeline/viewed-timeline-sync.ts`
 - App stream/timeline reducer: `packages/app/src/timeline/session-stream-reducers.ts`
 - Session wiring: `packages/app/src/contexts/session-context.tsx`