docs: migrate Skills to REST API

[Laurc2004]

What changed

• Migrates repository documentation and retained Skill references from MCP to the API-first REST contract.
• Keeps all nine Skill directories; the three research workflows now explicitly wait for a future supported REST API.
• Replaces the capability snapshot and validator with the fixed 14-path REST catalog.
• Links the public Agent Quickstart from both README versions.

Verification

• python3 .github/scripts/validate_skills.py
• python3 -m py_compile .github/scripts/validate_skills.py
• Repository-wide MCP reference scan passed
• git diff --check