ci(#1142): auto-close AIRC queue cards on canary merges

[joelteply]

Summary

• add Continuum's auto-close-queue-cards workflow for PRs merged into canary
• checkout CambrianTech/airc@canary inside the runner and invoke airc queue close-merged
• grant issue-write permissions so same-repo airc-queue cards can be marked merged and closed

Proof

• ruby -e "require 'yaml'; YAML.load_file('.github/workflows/auto-close-queue-cards.yml'); puts 'yaml ok'"
• git diff --check
• /Users/joelteply/.airc-src/airc queue close-merged docs(#1130): chat-to-AIRC migration proof gates #1141 --merge-sha 79e5fa7 --actor 'github-actions[continuum#1142]' --dry-run
• precommit hook: TypeScript build passed; browser tests skipped because local stack is down
• pre-push hook: TypeScript clean; ESLint 5530 errors vs baseline 6310; Rust/Docker skipped for workflow-only change

Closes #1142