Release: Slack CLI v4.13.0

Coding agents running slack --help now learn about the Slack MCP and Skills Plugin.

• We've added the CLI version to the frontmatter of the skill printed by the slack skill command, so a saved copy of the skill shows which release printed it.
• We've added a hint to the slack --help output for coding agents, suggesting they ask the user whether to install the Slack MCP and Skills Plugin. Nothing changes for people running the CLI themselves.