Add customer support screen share security skill

[YfengJ]

/claim #2411

Summary

• Add the customer-support-screen-share-security skill for customer support screen-share, co-browse, and remote-assist workflows.
• Cover consent and session binding, sensitive data shielding, privileged action controls, operator least privilege, replay/recording safety, exception paths, and regression evidence.
• Add vulnerable and benign JSON fixtures for the review gates.
• Quote existing ISO framework entries so the catalog index parses as YAML.

Verification

• ruby -ryaml -e 'idx = YAML.load_file("index.yaml"); files = idx.fetch("skills").map { |s| s.fetch("file") }; missing = files.reject { |f| File.exist?(f) }; raise "missing skill files: #{missing.join(", ")}" unless missing.empty?; count = idx.fetch("meta").fetch("skill_count"); raise "skill_count mismatch: #{count} vs #{files.size}" unless count == files.size; entry = idx.fetch("skills").find { |s| s.fetch("id") == "customer-support-screen-share-security" }; raise "missing index entry" unless entry; raise "wrong file" unless entry.fetch("file") == "skills/identity/customer-support-screen-share-security/SKILL.md"; puts "index ok: #{files.size} skills"'\n- ruby -e 'Dir["skills/**/*.md"].each { |f| body = File.read(f); count = body.scan(/```/).size; abort("unbalanced fences in #{f}: #{count}") if count.odd? }; puts "markdown fences ok"'\n- find tests -name '*.json' -print0 | xargs -0 -n1 jq empty\n- git diff --cached --check\n\nRequested bounty tier: Intermediate ($350). Payment details can be provided privately after maintainer acceptance.