Separate core/UI packages and streamline guidance

[Gabriel-Darbord]

Summary

• separate the headless MCP server package from the optional UI package
• rename MCP-Spec and MCP-Spec-Tests to MCP-UI and MCP-UI-Tests
• define separate Core, UI, Tests, and UI Tests groups; default loads all four
• update test and coverage configuration for the renamed UI packages
• reduce the README to purpose, installation, quick start, and documentation routes
• make Getting Started the canonical setup and load-group guide
• turn root and copyable AGENTS files into compact context-loading routers
• simplify the reusable Pharo workflow skills and correct stale tool guidance

Verification

• Pharo 13 full MCP-Tests and MCP-UI-Tests suite: 718 passed
• clean-image CI passes on Pharo 12, 13, and 14
• all five skill packages pass quick_validate.py
• copied templates installation validates
• all local Markdown links resolve
• no MCP-Spec package references remain
• git diff --check passes
• Git and live Iceberg working copies are clean