feat(gateway): private notice delivery and Slack format_message fixes

Adds platform-level private notice delivery abstraction so operational
messages (e.g. sethome prompt) can be sent ephemerally on Slack when
configured with `slack.notice_delivery: private`.

Changes:
- gateway/config.py: _normalize_notice_delivery() + GatewayConfig.get_notice_delivery()
  with per-platform config bridging
- gateway/platforms/base.py: send_private_notice() default implementation
  (falls through to send())
- gateway/platforms/slack.py: send_private_notice() via chat_postEphemeral
- gateway/run.py: _deliver_platform_notice() helper replaces direct
  adapter.send() for the sethome notice, with private→public fallback
- gateway/platforms/slack.py: app_mention handler now forwards to
  _handle_slack_message (safe due to ts-based dedup) instead of no-op pass,
  fixing edge-case Slack configs where mentions arrive only as app_mention
- gateway/platforms/slack.py format_message: negative lookbehind prevents
  markdown images (![]()) from becoming broken Slack links; italic regex
  now requires non-whitespace boundaries so 'a * b * c' stays literal

Based on PR #9340 by @probepark.

tests/gateway/test_config.py

@@ -213,6 +213,26 @@ class TestGatewayConfigRoundtrip:
         restored = GatewayConfig.from_dict({"always_log_local": "false"})
         assert restored.always_log_local is False
 
+    def test_get_notice_delivery_defaults_to_public(self):
+        config = GatewayConfig(
+            platforms={Platform.SLACK: PlatformConfig(enabled=True, token="***")}
+        )
+
+        assert config.get_notice_delivery(Platform.SLACK) == "public"
+
+    def test_get_notice_delivery_honors_platform_override(self):
+        config = GatewayConfig(
+            platforms={
+                Platform.SLACK: PlatformConfig(
+                    enabled=True,
+                    token="***",
+                    extra={"notice_delivery": "private"},
+                ),
+            }
+        )
+
+        assert config.get_notice_delivery(Platform.SLACK) == "private"
+
 
 class TestLoadGatewayConfig:
     def test_bridges_quick_commands_from_config_yaml(self, tmp_path, monkeypatch):
@@ -457,6 +477,22 @@ class TestLoadGatewayConfig:
 
         assert config.platforms[Platform.TELEGRAM].extra["disable_link_previews"] is True
 
+    def test_bridges_notice_delivery_from_config_yaml(self, tmp_path, monkeypatch):
+        hermes_home = tmp_path / ".hermes"
+        hermes_home.mkdir()
+        config_path = hermes_home / "config.yaml"
+        config_path.write_text(
+            "slack:\n"
+            "  notice_delivery: private\n",
+            encoding="utf-8",
+        )
+
+        monkeypatch.setenv("HERMES_HOME", str(hermes_home))
+
+        config = load_gateway_config()
+
+        assert config.get_notice_delivery(Platform.SLACK) == "private"
+
     def test_bridges_telegram_proxy_url_from_config_yaml(self, tmp_path, monkeypatch):
         hermes_home = tmp_path / ".hermes"
         hermes_home.mkdir()

tests/gateway/test_notice_delivery.py

@@ -0,0 +1,67 @@
+from unittest.mock import AsyncMock, MagicMock
+
+import pytest
+
+from gateway.config import GatewayConfig, Platform, PlatformConfig
+from gateway.platforms.base import SendResult
+from gateway.run import GatewayRunner
+from gateway.session import SessionSource
+
+
+def _make_source() -> SessionSource:
+    return SessionSource(
+        platform=Platform.SLACK,
+        chat_id="C123",
+        chat_type="channel",
+        user_id="U123",
+        thread_id="111.222",
+    )
+
+
+def _make_runner(extra=None):
+    runner = object.__new__(GatewayRunner)
+    runner.config = GatewayConfig(
+        platforms={
+            Platform.SLACK: PlatformConfig(enabled=True, token="***", extra=extra or {})
+        }
+    )
+    adapter = MagicMock()
+    adapter.send = AsyncMock(return_value=SendResult(success=True, message_id="public-1"))
+    adapter.send_private_notice = AsyncMock(return_value=SendResult(success=True, message_id="private-1"))
+    runner.adapters = {Platform.SLACK: adapter}
+    return runner, adapter
+
+
+@pytest.mark.asyncio
+async def test_deliver_platform_notice_uses_private_delivery_when_configured():
+    runner, adapter = _make_runner(extra={"notice_delivery": "private"})
+
+    await runner._deliver_platform_notice(_make_source(), "hello")
+
+    adapter.send_private_notice.assert_awaited_once_with(
+        "C123",
+        "U123",
+        "hello",
+        metadata={"thread_id": "111.222"},
+    )
+    adapter.send.assert_not_awaited()
+
+
+@pytest.mark.asyncio
+async def test_deliver_platform_notice_falls_back_to_public_when_private_fails():
+    runner, adapter = _make_runner(extra={"notice_delivery": "private"})
+    adapter.send_private_notice = AsyncMock(return_value=SendResult(success=False, error="nope"))
+
+    await runner._deliver_platform_notice(_make_source(), "hello")
+
+    adapter.send.assert_awaited_once_with("C123", "hello", metadata={"thread_id": "111.222"})
+
+
+@pytest.mark.asyncio
+async def test_deliver_platform_notice_uses_public_delivery_by_default():
+    runner, adapter = _make_runner()
+
+    await runner._deliver_platform_notice(_make_source(), "hello")
+
+    adapter.send.assert_awaited_once_with("C123", "hello", metadata={"thread_id": "111.222"})
+    adapter.send_private_notice.assert_not_awaited()

tests/gateway/test_slack.py

@@ -518,6 +518,28 @@ class TestSendDocument:
         sleep_mock.assert_awaited_once()
 
 
+class TestSendPrivateNotice:
+    @pytest.mark.asyncio
+    async def test_send_private_notice_uses_ephemeral_api(self, adapter):
+        adapter._app.client.chat_postEphemeral = AsyncMock(return_value={"message_ts": "123.456"})
+
+        result = await adapter.send_private_notice(
+            chat_id="C123",
+            user_id="U123",
+            content="private hello",
+            metadata={"thread_id": "1234567890.123456"},
+        )
+
+        assert result.success
+        adapter._app.client.chat_postEphemeral.assert_called_once_with(
+            channel="C123",
+            user="U123",
+            text="private hello",
+            mrkdwn=True,
+            thread_ts="1234567890.123456",
+        )
+
+
 # ---------------------------------------------------------------------------
 # TestSendVideo
 # ---------------------------------------------------------------------------
@@ -1315,6 +1337,16 @@ class TestFormatMessage:
         result = adapter.format_message("[link](https://x.com?a=1&b=2)")
         assert result == "<https://x.com?a=1&b=2|link>"
 
+    def test_markdown_image_does_not_create_broken_slack_link(self, adapter):
+        """Markdown image syntax should not become '!<url|alt>' in Slack."""
+        result = adapter.format_message("![alt](https://img.example.com/cat.png)")
+        assert result == "![alt](https://img.example.com/cat.png)"
+
+    def test_literal_asterisks_with_spaces_are_not_treated_as_italic(self, adapter):
+        """Asterisks used as plain delimiters should stay literal."""
+        result = adapter.format_message("a * b * c")
+        assert result == "a * b * c"
+
     def test_emoji_shortcodes_passthrough(self, adapter):
         """Emoji shortcodes like :smile: pass through unchanged."""
         assert adapter.format_message(":smile: hello :wave:") == ":smile: hello :wave:"