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.

gateway/platforms/base.py

@@ -1593,6 +1593,26 @@ class BasePlatformAdapter(ABC):
         """
         return SendResult(success=False, error="Not supported")
 
+    async def send_private_notice(
+        self,
+        chat_id: str,
+        user_id: Optional[str],
+        content: str,
+        reply_to: Optional[str] = None,
+        metadata: Optional[Dict[str, Any]] = None,
+    ) -> SendResult:
+        """Send a notice privately when the platform supports it.
+
+        The default implementation falls back to a normal send so callers can
+        use one code path across platforms.
+        """
+        return await self.send(
+            chat_id=chat_id,
+            content=content,
+            reply_to=reply_to,
+            metadata=metadata,
+        )
+
     async def send_typing(self, chat_id: str, metadata=None) -> None:
         """
         Send a typing indicator.