MCP: Add Prompts

[n7studios]

Summary

Adds MCP Prompts to the plugin - guided workflows an AI client can invoke by name, rather than the user having to know which tools and resources to string together.

Each prompt returns markdown telling the client what to read first, which tools to call, in what order, and to confirm with the user before writing. Five to start:

• kit/setup - confirm the site is connected, then set the default Form.
• kit/add-form - show a Form as a default, on a post, on a category, or inline.
• kit/restrict-content - gate a post or page behind a Product, Tag or Form.
• kit/configure-broadcasts-import - import Kit Broadcasts as WordPress posts.
• kit/audit - review the configuration and report gaps. Read only, and says so.

They're registered through a new convertkit_prompts filter, picked up by the Abilities API and exposed by the MCP server.

Testing

• MCPPromptsTest: Add integration tests for MCP prompts

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)