docs(agent-return-channel): record the idle-notification signature the defect presents as

The five mute teammates surfaced nine minutes after finishing - as
content-free idle notifications. That is akashic's "innholdslose idle-pings"
verbatim, and it arrived after the write-up was committed, so it is added
here as the field signature to recognise the defect by.

Two readable facts in it:
- `summary` is populated only for the teammate that called SendMessage; the
  five tool-less ones carry no content at all, not even an error. Presence of
  `summary` is the fastest mute-vs-delivering discriminator.
- The pings lag by minutes (ctrl-pong went idle 10s after spawn, notified ~9
  min later), so "no ping yet" is not evidence of work in progress and the
  ping is not a result. Judging liveness from these is how a finished run gets
  re-prompted nine times.

Suite unchanged: 1013 (1011/0/2).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014MfB5Ecp8vvGyou8uFGxBV

docs/agent-return-channel-defect.md

@@ -60,6 +60,35 @@ this is indistinguishable from an agent that stalled — so the natural response
 is to poke it with `SendMessage`, which produces more transcript that also
 never comes back. Nine reminders, zero replies.
 
+### The symptom signature, exactly
+
+The teammates do eventually surface — as content-free idle notifications,
+delivered long after the fact. Observed in this session, all six named agents
+(times UTC):
+
+```
+{"type":"idle_notification","from":"ctrl-pong",     "idleReason":"available"}   19:57:30
+{"type":"idle_notification","from":"ctrl-read",     "idleReason":"available"}   19:57:35
+{"type":"idle_notification","from":"pc-nooverride", "idleReason":"available"}   19:59:19
+{"type":"idle_notification","from":"pc-opus",       "idleReason":"available"}   19:59:24
+{"type":"idle_notification","from":"sg-opus",       "idleReason":"available"}   19:59:37
+{"type":"idle_notification","from":"ctrl-sendback", "idleReason":"available",
+                                   "summary":"[to main] PONG-VIA-SENDMESSAGE"}  20:06:16
+```
+
+This is akashic's "innholdsløse idle-pings", verbatim, and it is the field
+signature to recognise the defect by. Two things to read off it:
+
+- **`summary` is present only for the teammate that called SendMessage.** For
+  the five tool-less ones the notification carries no content at all — not a
+  truncated result, not an error, nothing. The presence or absence of `summary`
+  is the fastest way to tell a delivering teammate from a mute one.
+- **The idle notifications lag by minutes.** `ctrl-pong` went idle 10 seconds
+  after spawn; its notification arrived ~9 minutes later. So "no idle ping yet"
+  is not evidence the agent is still working, and the ping, when it comes, is
+  not a result. Judging liveness from these is how a completed run gets
+  re-prompted nine times.
+
 **The work is not lost.** It is on disk at
 `~/.claude/projects/<project-slug>/<session-id>/subagents/agent-a<name>-<hash>.jsonl`.
 The final assistant text block in that file is the answer the orchestrator never