Add Copilot issue Slack notifier workflow to release-assets

[Copilot]

Sets up the same issues.opened Slack notifier from github/support-squad-copilot in this repository, targeting #support-squad-copilot-shiftlog with the message Review release and assign SME.

Workflow (.github/workflows/copilot-issue-slack-notifier.yml)

• Triggers on issues.opened
• Matches whole word copilot (case-insensitive) in title or body via /\bcopilot\b/i; skips silently on no match
• Safely coerces null/empty body to '' before matching
• POSTs to Slack via actions/github-script@v7 using Node's built-in https — no extra dependencies
• Payload includes channel, fixed text, and an attachment with issue title (linked), author login, and repo name
• Reads webhook URL from SLACK_WEBHOOK_URL repository secret

README

• Documents keyword matching behaviour, secret setup path, and a test matrix covering match/no-match/partial-word/empty-body scenarios

Required before merge

Add SLACK_WEBHOOK_URL under Settings → Secrets and variables → Actions pointing at the #support-squad-copilot-shiftlog incoming webhook URL.