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

container/skills/slack-formatting/SKILL.md

@@ -0,0 +1,94 @@
+---
+name: slack-formatting
+description: Format messages for Slack using mrkdwn syntax. Use when responding to Slack channels (folder starts with "slack_" or JID contains slack identifiers).
+---
+
+# Slack Message Formatting (mrkdwn)
+
+When responding to Slack channels, use Slack's mrkdwn syntax instead of standard Markdown.
+
+## How to detect Slack context
+
+Check your group folder name or workspace path:
+- Folder starts with `slack_` (e.g., `slack_engineering`, `slack_general`)
+- Or check `/workspace/group/` path for `slack_` prefix
+
+## Formatting reference
+
+### Text styles
+
+| Style | Syntax | Example |
+|-------|--------|---------|
+| Bold | `*text*` | *bold text* |
+| Italic | `_text_` | _italic text_ |
+| Strikethrough | `~text~` | ~strikethrough~ |
+| Code (inline) | `` `code` `` | `inline code` |
+| Code block | ` ```code``` ` | Multi-line code |
+
+### Links and mentions
+
+```
+<https://example.com|Link text>     # Named link
+<https://example.com>                # Auto-linked URL
+<@U1234567890>                       # Mention user by ID
+<#C1234567890>                       # Mention channel by ID
+<!here>                              # @here
+<!channel>                           # @channel
+```
+
+### Lists
+
+Slack supports simple bullet lists but NOT numbered lists:
+
+```
+• First item
+• Second item
+• Third item
+```
+
+Use `•` (bullet character) or `- ` or `* ` for bullets.
+
+### Block quotes
+
+```
+> This is a block quote
+> It can span multiple lines
+```
+
+### Emoji
+
+Use standard emoji shortcodes: `:white_check_mark:`, `:x:`, `:rocket:`, `:tada:`
+
+## What NOT to use
+
+- **NO** `##` headings (use `*Bold text*` for headers instead)
+- **NO** `**double asterisks**` for bold (use `*single asterisks*`)
+- **NO** `[text](url)` links (use `<url|text>` instead)
+- **NO** `1.` numbered lists (use bullets with numbers: `• 1. First`)
+- **NO** tables (use code blocks or plain text alignment)
+- **NO** `---` horizontal rules
+
+## Example message
+
+```
+*Daily Standup Summary*
+
+_March 21, 2026_
+
+• *Completed:* Fixed authentication bug in login flow
+• *In Progress:* Building new dashboard widgets
+• *Blocked:* Waiting on API access from DevOps
+
+> Next sync: Monday 10am
+
+:white_check_mark: All tests passing | <https://ci.example.com/builds/123|View Build>
+```
+
+## Quick rules
+
+1. Use `*bold*` not `**bold**`
+2. Use `<url|text>` not `[text](url)`
+3. Use `•` bullets, avoid numbered lists
+4. Use `:emoji:` shortcodes
+5. Quote blocks with `>`
+6. Skip headings — use bold text instead