fix(sync): guard child-session discovery against overlapping pulls

Child discovery now pages through the whole session list, so one pull can
take longer than the 15s watchdog interval. Two overlapping pulls each read
the store before either commits, so both appended the same newly found child
and the directory store ended up with duplicate rows. Keep one pull per
directory in flight, mirroring the status-poll guard.

packages/ui/src/sync/sync-context.tsx

@@ -366,6 +366,12 @@ const pendingSessionMaterializations = new Map<string, PendingSessionMaterializa
 // the same directory.
 const statusPollingDirectories = new Set<string>()
 
+// Directories with a child-session discovery pull in flight. Discovery now
+// pages through the session list, so one pull can outlast the watchdog
+// interval; two overlapping pulls would each snapshot the same pre-commit
+// session list and append the same child twice.
+const childDiscoveryDirectories = new Set<string>()
+
 // Deferred completion polls awaiting their delay, keyed by directory+session so
 // a burst of completing messages schedules one check.
 const pendingMessageCompletionPolls = new Map<string, ReturnType<typeof setTimeout>>()
@@ -2513,6 +2519,8 @@ export function SyncProvider(props: {
       parentSessionIds: string[],
     ) => {
       if (parentSessionIds.length === 0) return
+      if (childDiscoveryDirectories.has(directory)) return
+      childDiscoveryDirectories.add(directory)
       try {
         // Paginated so directories with > pageSize sessions are fully
         // discovered; a single 200-record page silently truncated the list and
@@ -2557,6 +2565,8 @@ export function SyncProvider(props: {
         }
       } catch {
         // Best-effort — next tick will retry.
+      } finally {
+        childDiscoveryDirectories.delete(directory)
       }
     }