fix(slack): quiet Slack display defaults — no heartbeat/busy-ack breadcrumbs in channels

Slack posts are durable workspace messages, not an ephemeral terminal
status area. Default long_running_notifications and busy_ack_detail to
off for Slack so long-running agent work does not leave permanent
operational breadcrumbs like 'Working — 9 min — iteration 12/90' in
channels. Both remain opt-in per platform via
display.platforms.slack.*.

Also covers the platform-generic shutdown-notification mute path with a
regression test (gateway_restart_notification=false must suppress both
the active-session interruption notice and the home-channel copy).

Salvaged from #69028 (quiet-defaults half only). The PR's other half —
the channel_session_scope_channels session-scoping feature — is a new
config feature outside this log-noise cluster and overlaps the session
scoping territory reworked by merged wave-1/2 Slack session work; it is
deliberately not taken here.

gateway/display_config.py

@@ -140,7 +140,12 @@ _PLATFORM_DEFAULTS: dict[str, dict[str, Any]] = {
     # Tier 2 — edit support, often customer/workspace channels
     # Slack: tool_progress off by default — Bolt posts cannot be edited like CLI;
     # "new"/"all" spam permanent lines in channels (hermes-agent#14663).
-    "slack":           {**_TIER_MEDIUM, "tool_progress": "off"},
+    "slack":           {
+        **_TIER_MEDIUM,
+        "tool_progress": "off",
+        "long_running_notifications": False,
+        "busy_ack_detail": False,
+    },
     "mattermost":      _TIER_MEDIUM,
     "matrix":          _TIER_MEDIUM,
     "feishu":          _TIER_MEDIUM,

tests/gateway/test_display_config.py

@@ -328,6 +328,14 @@ class TestPlatformDefaults:
         assert resolve_display_setting({}, "discord", "long_running_notifications") is True
         assert resolve_display_setting({}, "discord", "busy_ack_detail") is True
 
+    def test_slack_workspace_chatter_defaults(self):
+        """Slack should not leave permanent heartbeat/debug breadcrumbs in channels."""
+        from gateway.display_config import resolve_display_setting
+
+        assert resolve_display_setting({}, "slack", "tool_progress") == "off"
+        assert resolve_display_setting({}, "slack", "long_running_notifications") is False
+        assert resolve_display_setting({}, "slack", "busy_ack_detail") is False
+
     def test_telegram_mobile_chatter_can_opt_in(self):
         """Per-platform config can re-enable Telegram busy-ack detail
         and re-disable the kept-on defaults."""

tests/gateway/test_restart_notification.py

@@ -665,6 +665,27 @@ async def test_shutdown_notifications_use_cached_live_thread_source_when_origin_
     )
 
 
+@pytest.mark.asyncio
+async def test_shutdown_notifications_are_fully_muted_when_flag_disabled():
+    runner, adapter = make_restart_runner()
+    source = make_restart_source(chat_id="active-42", chat_type="group", thread_id="topic-7")
+    session_key = build_session_key(source)
+
+    runner.config.platforms[Platform.TELEGRAM].gateway_restart_notification = False
+    runner.config.platforms[Platform.TELEGRAM].home_channel = HomeChannel(
+        platform=Platform.TELEGRAM,
+        chat_id="home-42",
+        name="Ops Home",
+    )
+    runner._running_agents[session_key] = object()
+    runner.session_store._entries[session_key] = MagicMock(origin=source)
+    adapter.send = AsyncMock()
+
+    await runner._notify_active_sessions_of_shutdown()
+
+    adapter.send.assert_not_awaited()
+
+
 @pytest.mark.asyncio
 async def test_restart_shutdown_notification_anchors_telegram_dm_topic():
     runner, adapter = make_restart_runner()