feat(jobs): push background-job completion notifications via per-context queue

- add NotificationQueue/SystemNotification: every context owns a fresh
  queue (children never inherit the parent's, avoiding first-drainer-wins
  races between transcripts)
- job tasks push job_completed/job_failed events on completion, failure,
  and timeout; a panic skips the push and is surfaced by the guardrail's
  finished-handle enumeration and collect's JoinError mapping instead
- events for jobs already collected or cancelled are dropped at drain time
  by filtering against live supervisor registration
- replace inject_escalation_notification with single-pass
  merge_system_channel: pending_escalations (root-only) ordered before
  system_notifications (any depth) on the last tool result of a batch;
  byte-identical output when notifications are empty, proven by the
  unmodified pre-merger characterization tests

src/supervisor/notification.rs

@@ -0,0 +1,128 @@
+use fmt::{Debug, Formatter};
+use serde_json::{Value, json};
+use std::fmt;
+
+/// One background-task completion event, delivered to the context that
+/// started the task by merging a `system_notifications` entry onto the last
+/// tool result of a batch.
+#[derive(Clone)]
+pub struct SystemNotification {
+    pub event: &'static str,
+    pub id: String,
+    pub tool_or_agent: String,
+    pub status: &'static str,
+    pub next_action: String,
+}
+
+impl SystemNotification {
+    pub fn to_value(&self) -> Value {
+        json!({
+            "event": self.event,
+            "id": self.id,
+            "tool_or_agent": self.tool_or_agent,
+            "status": self.status,
+            "next_action": self.next_action,
+        })
+    }
+}
+
+pub fn job_notification(id: &str, tool: &str, success: bool) -> SystemNotification {
+    SystemNotification {
+        event: if success {
+            "job_completed"
+        } else {
+            "job_failed"
+        },
+        id: id.to_string(),
+        tool_or_agent: tool.to_string(),
+        status: if success { "success" } else { "failed" },
+        next_action: format!("job__collect --id {id} for output"),
+    }
+}
+
+/// Completion events for background work started by ONE context. Unlike the
+/// escalation queue (shared, root-owned), every context owns a fresh queue:
+/// a queue shared between parent and child would race their drains and
+/// deliver one context's events into the other's transcript.
+pub struct NotificationQueue {
+    pending: parking_lot::Mutex<Vec<SystemNotification>>,
+}
+
+impl NotificationQueue {
+    pub fn new() -> Self {
+        Self {
+            pending: parking_lot::Mutex::new(Vec::new()),
+        }
+    }
+
+    pub fn push(&self, notification: SystemNotification) {
+        self.pending.lock().push(notification);
+    }
+
+    pub fn drain(&self) -> Vec<SystemNotification> {
+        std::mem::take(&mut *self.pending.lock())
+    }
+}
+
+impl Default for NotificationQueue {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl Debug for NotificationQueue {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        let count = self.pending.lock().len();
+        f.debug_struct("NotificationQueue")
+            .field("pending_count", &count)
+            .finish()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn job_notification_success_shape() {
+        let event = job_notification("job_a1b2", "execute_command", true);
+        assert_eq!(
+            event.to_value(),
+            json!({
+                "event": "job_completed",
+                "id": "job_a1b2",
+                "tool_or_agent": "execute_command",
+                "status": "success",
+                "next_action": "job__collect --id job_a1b2 for output",
+            })
+        );
+    }
+
+    #[test]
+    fn job_notification_failure_shape() {
+        let event = job_notification("job_a1b2", "execute_command", false);
+        assert_eq!(event.event, "job_failed");
+        assert_eq!(event.status, "failed");
+        assert_eq!(event.next_action, "job__collect --id job_a1b2 for output");
+    }
+
+    #[test]
+    fn drain_empties_queue_and_preserves_order() {
+        let queue = NotificationQueue::new();
+        queue.push(job_notification("job_1", "execute_command", true));
+        queue.push(job_notification("job_2", "execute_command", false));
+
+        let drained = queue.drain();
+
+        assert_eq!(drained.len(), 2);
+        assert_eq!(drained[0].id, "job_1");
+        assert_eq!(drained[1].id, "job_2");
+        assert!(queue.drain().is_empty());
+    }
+
+    #[test]
+    fn drain_on_empty_queue_is_a_noop() {
+        let queue = NotificationQueue::default();
+        assert!(queue.drain().is_empty());
+    }
+}