feat: add GTM AI marketplace skills

[dfacher]

Summary

• Add 20 GTM AI marketplace workflows as actual plugin skills under skills/*/SKILL.md
• Align the new skills with the full documented GTM AI MCP tool set from https://gtm.ai/docs/mcp/tools
• Fix stale recommended-contact tool naming to use find_recommended_contacts
• Fix recommend-contacts frontmatter name mismatch
• Update the README skills index

Validation

• Validated all 20 new skills include frontmatter, Input, MCP Tool Selection, Workflow, Output Format, and Quality Bar sections
• Confirmed all 15 documented MCP tools are represented across the skill set
• Confirmed no remaining stale get_recommended_contacts references
• Confirmed all 20 new skills are referenced in README