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.
0ab2d752ff