feat: dynamic prompts CRUD — add/delete custom prompts from admin UI
- Extract built-in prompts to getBuiltinPrompts()
- renderPromptsTab() now merges builtins with custom prompts from WP option
- Add New Prompt form: title, tag/industry, description, content
- Custom prompt cards show green "Custom" badge + Delete button
- handleApiKeyActions() handles add_prompt and delete_prompt actions
- Custom prompts stored in asktam_mcp_custom_prompts WP option
- Nonce-protected, sanitized, redirect-after-POST pattern
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