chore: migrate skills to agents layout

[LIghtJUNction]

Summary

• move public skills under .agents/skills and add AGENT.md
• replace the separate researchskills-extract package with a ResearchSkills meta skill
• simplify README contribution/install flow around npx/bunx skills add and update path references
• update CODEOWNERS, PR templates, maintainer onboarding, validators, and docs for .agents/skills

Verification

• python utils/tools/validate.py \
• node utils/scripts/generate-taxonomy.js --out /tmp/researchskills-taxonomy.json
• rg -n 'researchskills-extract|researchskills-convert|@scienceintelligence/researchskills-extract|npm install -g' .

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