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

Summary

The slack:slack-messaging skill provides helpful guidance on Slack's mrkdwn formatting, but it's missing critical information about blank line requirements that cause visual rendering issues in Slack.

Issue

Slack's mrkdwn format requires blank lines before certain elements, or they will run together on the same line instead of appearing on separate lines:

1. Code blocks (especially when used for formatting tables)
2. Horizontal separators/dividers

Without blank lines, the preceding text and these elements merge into a single line, creating broken formatting.

Visual Example

❌ Without blank line (broken):

When you write:

_Table (using preformatted text):_
\`\`\`
┌─────────────┬──────────┬─────────┐
│ Command     │ Purpose  │ Status  │
└─────────────┴──────────┴─────────┘
\`\`\`

Slack renders it as:

Table (using preformatted text): ┌─────────────┬──────────┬─────────┐
│ Command     │ Purpose  │ Status  │
└─────────────┴──────────┴─────────┘

The label and first line of the table run together!

✅ With blank line (correct):

When you write:

_Table (using preformatted text):_

\`\`\`
┌─────────────┬──────────┬─────────┐
│ Command     │ Purpose  │ Status  │
└─────────────┴──────────┴─────────┘
\`\`\`

Slack renders it properly:

Table (using preformatted text):

┌─────────────┬──────────┬─────────┐
│ Command     │ Purpose  │ Status  │
└─────────────┴──────────┴─────────┘

The label and table appear on separate lines as intended.

Same Issue with Horizontal Separators

❌ Without blank line:

_Horizontal separator:_
───────────────────────

Renders as: Horizontal separator:─────────────────────── (merged on one line)

✅ With blank line:

_Horizontal separator:_

───────────────────────

Renders correctly with the separator on its own line.

Suggested Fix

Add this to the "Common Mistakes to Avoid" or create a new "Formatting Tips" section:

• Always add a blank line before code blocks - without it, the preceding text and code block will merge into a single line
• Add blank lines before horizontal separators - otherwise they'll run together with the previous text

Impact

This causes messages created via slack_send_message and slack_send_message_draft to have broken formatting that looks unprofessional and is hard to read.