Make tombstone confirm dedupe add-and-fork atomic under interruption

The pendingTombstoneConfirms entry was added before the detached confirm
fiber was forked, with interruptible yield points in between. If the
publishing fiber was interrupted in that window the entry leaked, and the
child ensuring finalizer that owns the delete never existed, permanently
suppressing tombstone confirmation for that thread. Perform the
check-add-fork sequence inside a single uninterruptible region so the set
entry exists only when the confirm fiber (and its cleanup) is guaranteed.

Author: cursoragent

apps/server/src/relay/AgentAwarenessRelay.ts

@@ -432,20 +432,30 @@ export const make = Effect.gen(function* () {
       // immediately deletes the thread from the lock-screen card mid-
       // conversation. Defer, re-resolve, and only tombstone if it holds;
       // genuinely deleted threads still propagate a few seconds later.
-      if (pendingTombstoneConfirms.has(threadId)) {
-        return;
-      }
-      pendingTombstoneConfirms.add(threadId);
-      yield* Effect.logInfo("agent activity tombstone deferred pending confirmation", {
-        environmentId,
-        threadId,
-        reason: snapshot.reason,
-        shell: describeThreadShellForAwareness(thread),
-      });
-      yield* Effect.forkDetach(
-        confirmTombstoneLater(threadId).pipe(
-          Effect.ensuring(Effect.sync(() => pendingTombstoneConfirms.delete(threadId))),
-        ),
+      // Check-add-fork must be atomic under interruption: a set entry without
+      // a forked confirm fiber (whose `ensuring` owns the delete) would
+      // suppress tombstone confirmation for this thread forever.
+      yield* Effect.uninterruptible(
+        Effect.suspend(() => {
+          if (pendingTombstoneConfirms.has(threadId)) {
+            return Effect.void;
+          }
+          pendingTombstoneConfirms.add(threadId);
+          return Effect.logInfo("agent activity tombstone deferred pending confirmation", {
+            environmentId,
+            threadId,
+            reason: snapshot.reason,
+            shell: describeThreadShellForAwareness(thread),
+          }).pipe(
+            Effect.andThen(
+              Effect.forkDetach(
+                confirmTombstoneLater(threadId).pipe(
+                  Effect.ensuring(Effect.sync(() => pendingTombstoneConfirms.delete(threadId))),
+                ),
+              ),
+            ),
+          );
+        }),
       );
       return;
     }