fix(desktop): scope live observer feed to active channel on open

Both openAgentSession and selectAgentSession were calling
setOpenAgentSessionChannelId(channelId ?? null). When the activity-list
opener supplies no explicit channelId, the null propagated into
sessionChannelId, which caused scopeByChannel to skip filtering entirely
(its null fast-path returns all items). Combined with the shared per-pubkey
store, this let every channel's live frames appear in any channel's feed.

Fix: fall back to activeChannelId in both callbacks so opening from within
a channel always resolves a non-null scope key. activeChannelId is already
in the useChannelAgentSessions parameter list; add it to both callbacks'
dependency arrays.

Also fix the archive-load enabled gate: useLoadArchivedObserverEvents was
gated on isLive, so an idle agent's channel showed no archived observer
history. Change the enable condition to Boolean(sessionChannelId) so
archive history loads whenever a channel scope is resolved, regardless of
whether the agent is currently running.

Co-authored-by: Will Pfleger <pfleger.will@gmail.com>
Signed-off-by: Will Pfleger <pfleger.will@gmail.com>

desktop/src/features/channels/ui/AgentSessionThreadPanel.tsx

@@ -131,7 +131,14 @@ export function AgentSessionThreadPanel({
       : `Last updated ${new Date(latestActivityAt).toLocaleString()}`;
 
   const { fetchOlderArchived, hasOlderArchived } =
-    useLoadArchivedObserverEvents(isLive, sessionChannelId ?? null);
+    useLoadArchivedObserverEvents(
+      // Archive history must load regardless of live status — an idle agent's
+      // channel should still show its archived observer history. Enable whenever
+      // there is a resolved sessionChannelId (the hook's owner_p guard handles
+      // the case where no save subscription exists).
+      Boolean(sessionChannelId),
+      sessionChannelId ?? null,
+    );
 
   useLoadOlderOnScroll({
     fetchOlder: fetchOlderArchived,

desktop/src/features/channels/ui/useChannelAgentSessions.ts

@@ -220,9 +220,14 @@ export function useChannelAgentSessions({
       setThreadReplyTargetId(null);
       setChannelManagementOpen(false);
       setOpenAgentSessionPubkey(pubkey);
-      setOpenAgentSessionChannelId(channelId ?? null);
+      // Fall back to activeChannelId so opening from within a channel always
+      // scopes the panel to that channel — even when no explicit channelId is
+      // supplied (e.g. activity-list click). Without this, a null channelId
+      // bypasses scopeByChannel and lets all channels' live frames through.
+      setOpenAgentSessionChannelId(channelId ?? activeChannelId ?? null);
     },
     [
+      activeChannelId,
       isAgentSessionOpen,
       openThreadHeadId,
       profilePanelPubkey,
@@ -260,9 +265,11 @@ export function useChannelAgentSessions({
   const selectAgentSession = React.useCallback(
     (pubkey: string, channelId?: string | null) => {
       setOpenAgentSessionPubkey(pubkey);
-      setOpenAgentSessionChannelId(channelId ?? null);
+      // Same fallback as openAgentSession: use activeChannelId when the caller
+      // omits channelId, so the panel is always scoped to the current channel.
+      setOpenAgentSessionChannelId(channelId ?? activeChannelId ?? null);
     },
-    [setOpenAgentSessionChannelId, setOpenAgentSessionPubkey],
+    [activeChannelId, setOpenAgentSessionChannelId, setOpenAgentSessionPubkey],
   );
 
   const openThreadAndCloseAgentSession = React.useCallback(