fix(ci): authorize agent labels by trigger actor

[devi-labs]

Summary

• authorize label-triggered agent workflows by the actor who applied the label using repo permission (admin/maintain/write)
• stop using issue author association as the label-trigger gate
• switch agent prompts to use gh api for issue details so author_association is available without invalid gh issue view fields

Testing

• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/agent-triage.yml"); YAML.load_file(".github/workflows/agent-fix.yml"); puts "yaml ok"'
• gh api repos/Shopify/flash-list/collaborators/devi-labs/permission --jq .permission