[FEATURE] Improve slack:slack-messaging skill - document blank line requirements

Feature Request

The slack:slack-messaging skill should document that Slack's mrkdwn requires blank lines before code blocks and horizontal separators.

Problem

When Claude Code creates Slack messages without blank lines before these elements, Slack renders them incorrectly by merging the preceding text with the code block/separator on the same line.

Example of broken output:

_Table:_
\`\`\`
┌─────┬─────┐
│ A   │ B   │
└─────┴─────┘
\`\`\`

Renders in Slack as:

Table: ┌─────┬─────┐
│ A   │ B   │
└─────┴─────┘

The label and table merge into one line.

Solution

Add to the skill documentation:

Formatting Tips:

• Add a blank line before code blocks - without it, preceding text merges with the code block
• Add a blank line before horizontal separators - same issue

Impact

This will help Claude Code generate properly formatted Slack messages via slack_send_message and slack_send_message_draft tools.

---

Discovered while testing Slack integration in Claude Code.