feat: add advanced frontmatter config for Claude agent definitions

Support all Claude agent frontmatter fields (color, permissionMode, memory,
maxTurns, background, isolation, tools, disallowedTools, skills, mcpServers,
hooks) with MCP server picker, hooks editor, and progressive disclosure UI.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: catlog22