feat: add Slack formatting skill for NanoClaw agents

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

groups/global/CLAUDE.md

@@ -49,10 +49,28 @@ When you learn something important:
 
 ## Message Formatting
 
-NEVER use markdown. Only use WhatsApp/Telegram formatting:
-- *single asterisks* for bold (NEVER **double asterisks**)
-- _underscores_ for italic
-- • bullet points
-- ```triple backticks``` for code
+Format messages based on the channel you're responding to. Check your group folder name:
 
-No ## headings. No [links](url). No **double stars**.
+### Slack channels (folder starts with `slack_`)
+
+Use Slack mrkdwn syntax. Run `/slack-formatting` for the full reference. Key rules:
+- `*bold*` (single asterisks)
+- `_italic_` (underscores)
+- `<https://url|link text>` for links (NOT `[text](url)`)
+- `•` bullets (no numbered lists)
+- `:emoji:` shortcodes
+- `>` for block quotes
+- No `##` headings — use `*Bold text*` instead
+
+### WhatsApp/Telegram channels (folder starts with `whatsapp_` or `telegram_`)
+
+- `*bold*` (single asterisks, NEVER **double**)
+- `_italic_` (underscores)
+- `•` bullet points
+- ` ``` ` code blocks
+
+No `##` headings. No `[links](url)`. No `**double stars**`.
+
+### Discord channels (folder starts with `discord_`)
+
+Standard Markdown works: `**bold**`, `*italic*`, `[links](url)`, `# headings`.