fix(gateway): /sethome must not persist Slack's synthetic per-message session thread as the home target

Third lane of the same contract (found in live staging validation):
/sethome run as a top-level relay-fronted Slack DM message captured the
adapter's session-keying thread stamp (the /sethome message's own id)
into the persisted HomeChannel.thread_id and its legacy env mirror.
Every bare-platform delivery (deliver="slack") then resolved home chat +
home thread and landed inside the ephemeral thread around the old
/sethome message. Extracted _home_thread_from_source with the same
synthetic-stamp recognition as cron origin capture; a /sethome run
inside a genuine thread keeps that thread as the home target. Users
repair an already-poisoned home target by rerunning /sethome.

gateway/slash_commands.py

@@ -98,6 +98,31 @@ def _model_switch_skew_guard() -> Optional[str]:
     )
 
 
+def _home_thread_from_source(source) -> Optional[str]:
+    """The thread id /sethome should persist on the home target, or None.
+
+    Slack thread-per-message session keying stamps a top-level message's own
+    id as ``source.thread_id`` (a session KEY, not a durable location).
+    Persisting it would pin the HOME target itself to the ephemeral thread
+    spawned around the /sethome message — every bare-platform delivery
+    (``deliver="slack"``) would then land in that thread forever. Same
+    recognition as cron origin capture: a Slack thread id equal to the
+    message's own id is synthetic. A /sethome run inside a genuine thread
+    (thread id = the parent's id, not this message's own) keeps that thread
+    as the home target.
+    """
+    thread_id = getattr(source, "thread_id", None)
+    if not thread_id:
+        return None
+    if (
+        getattr(source, "platform", None) == Platform.SLACK
+        and getattr(source, "message_id", None)
+        and str(thread_id) == str(source.message_id)
+    ):
+        return None
+    return str(thread_id)
+
+
 class GatewaySlashCommandsMixin:
     """In-session slash-command handlers for GatewayRunner."""
 
@@ -3020,7 +3045,7 @@ class GatewaySlashCommandsMixin:
                     error="Relay does not authenticate this logical home target",
                 )
 
-        thread_id = source.thread_id
+        thread_id = _home_thread_from_source(source)
         home = HomeChannel(
             platform=source.platform,
             chat_id=str(chat_id),

tests/gateway/test_sethome_synthetic_thread.py

@@ -0,0 +1,48 @@
+"""/sethome must not persist Slack's synthetic per-message session thread.
+
+Live repro (relay-fronted Slack staging, 2026-08-13): /sethome run as a
+top-level DM message captured source.thread_id — which the relay adapter had
+stamped with the /sethome message's OWN id for session keying — into the
+persisted HomeChannel. Every bare-platform delivery (deliver="slack") then
+resolved home chat + home thread and landed inside the ephemeral thread
+spawned around the old /sethome message.
+
+Same contract as cron origin capture: a Slack thread id equal to the
+message's own id is a synthetic session key, never a durable location.
+"""
+
+from types import SimpleNamespace
+
+from gateway.config import Platform
+from gateway.slash_commands import _home_thread_from_source
+
+
+def _source(platform=Platform.SLACK, thread_id=None, message_id=None):
+    return SimpleNamespace(
+        platform=platform, thread_id=thread_id, message_id=message_id
+    )
+
+
+class TestHomeThreadFromSource:
+    def test_synthetic_slack_thread_dropped(self):
+        """Top-level /sethome: stamped thread == own message id -> None."""
+        src = _source(thread_id="1755043010.123456", message_id="1755043010.123456")
+        assert _home_thread_from_source(src) is None
+
+    def test_genuine_slack_thread_kept(self):
+        """/sethome inside a real thread keeps that thread as home target."""
+        src = _source(thread_id="1755040000.000100", message_id="1755043010.123456")
+        assert _home_thread_from_source(src) == "1755040000.000100"
+
+    def test_no_thread_returns_none(self):
+        assert _home_thread_from_source(_source()) is None
+
+    def test_no_message_id_keeps_thread(self):
+        """Without a message id to compare, never guess: keep the thread."""
+        src = _source(thread_id="1755040000.000100", message_id=None)
+        assert _home_thread_from_source(src) == "1755040000.000100"
+
+    def test_non_slack_platform_untouched(self):
+        """Telegram forum topics legitimately reuse ids; rule is Slack-scoped."""
+        src = _source(platform=Platform.TELEGRAM, thread_id="2203", message_id="2203")
+        assert _home_thread_from_source(src) == "2203"