Add a new skill that teaches agents how to format messages using Slack's
mrkdwn syntax. Updates agent CLAUDE.md files to detect channel type from
folder name prefix and use appropriate formatting.
- container/skills/slack-formatting/SKILL.md: comprehensive mrkdwn reference
- groups/global/CLAUDE.md: channel-aware formatting instructions
- groups/main/CLAUDE.md: same, plus emoji shortcode examples
https://claude.ai/code/session_01W44WtL2gRETr9YBB6h62YM
