fix: escape issue title in Slack notification to prevent JSON breakage

[MaxwellCalkin]

Summary

• Issue titles containing ", \, newlines, or other JSON-special characters break the slack-issue-notification.yml payload, causing Slack notifications to silently fail
• Replaces inline ${{ }} expression interpolation with a jq-based approach that properly JSON-escapes all user-supplied values (issue.title, issue.user.login, etc.) via --arg before embedding them in the payload
• Preserves the same Slack message format, channel, and pinned action version

Problem

The current workflow interpolates ${{ github.event.issue.title }} directly into a JSON string literal:

"text": "*New Issue:* <url|#123 ${{ github.event.issue.title }}>"

If someone opens an issue titled Bug: "undefined" is not a function, the expanded payload becomes:

"text": "*New Issue:* <url|#123 Bug: "undefined" is not a function>"

This is invalid JSON. The slackapi/slack-github-action action fails to parse it and the notification is never sent.

Fix

Event data is passed through environment variables into a run step that uses jq --arg to build the payload. jq handles all JSON escaping automatically, so any title — including those with quotes, backslashes, angle brackets, or newlines — produces valid JSON.

Test plan

• Open a test issue with a normal title and verify the Slack notification arrives
• Open a test issue with special characters in the title (e.g. Test "quotes" and \backslash) and verify the notification still arrives with the correct title

AI Disclosure

This PR was authored by Claude Opus 4.6 (Anthropic), an AI agent operated by Maxwell Calkin (@MaxwellCalkin).