feat(ci): notify #team-fairies when backwards-compat e2e fails on nightly (#23003)

## Summary

`ci-compat-e2e` is `continue-on-error: true` on nightly tags (see
`.github/workflows/ci3.yml` line 316), so when it fails the workflow
stays green and nobody finds out unless they manually open the run.
Today's nightly compat failure (contract-artifacts) is the surfacing
case.

Wraps `bootstrap_ec2 "./bootstrap.sh ci-compat-e2e"` in `ci.sh` so that
on a `-nightly.` tag a failing run posts to `#team-fairies` via the
existing `ci3/slack_notify` helper, then exits with the captured rc (so
GH job result is unchanged).

Stable/RC tags already block the release pipeline on this job and
PR-label runs are interactive, so the notification is intentionally
limited to nightlies.

## Targeting

PR targets `v4-next` because `ci-compat-e2e` only lives on `v4-next`
(not on `next`).

## Notes

A workflow-level step (`if: failure() && contains(github.ref_name,
'-nightly.')` in the `ci-compat-e2e` job) would be slightly more
idiomatic and mirror the existing "Notify Slack on backport CI failure"
step at lines 116-130. This session was started without `ci-allow`, so
`.github/workflows/ci3.yml` is off-limits to `create_pr`; the
`ci.sh`-level change is functionally equivalent. An exact workflow-level
patch is documented in
https://gist.github.com/AztecBot/1004e4a07b57807329b334b043c17d12 in
case the team prefers to apply it that way later.

Closes
[F-632](https://linear.app/aztec-labs/issue/F-632/notify-team-fairies-channel-when-backwards-compat-fails).

ClaudeBox log: https://claudebox.work/s/a0aa5c1e0994ac85?run=1

Author: benesjan

ci.sh

@@ -265,7 +265,15 @@ case "$cmd" in
     export CI_DASHBOARD="releases"
     export JOB_ID="x-compat-e2e"
     export AWS_SHUTDOWN_TIME=300
-    bootstrap_ec2 "./bootstrap.sh ci-compat-e2e"
+    rc=0
+    bootstrap_ec2 "./bootstrap.sh ci-compat-e2e" || rc=$?
+    # On nightly tags compat-e2e is non-blocking (continue-on-error in ci3.yml), so
+    # failures otherwise go unnoticed. Notify #team-fairies so they get picked up.
+    if [ "$rc" -ne 0 ] && [[ "${REF_NAME:-}" == *-nightly.* ]]; then
+      run_url="https://github.com/${GITHUB_REPOSITORY:-AztecProtocol/aztec-packages}/actions/runs/${GITHUB_RUN_ID:-unknown}"
+      "$ci3/slack_notify" "Backwards compatibility e2e tests FAILED on nightly tag <${run_url}|${REF_NAME}>" "#team-fairies"
+    fi
+    exit "$rc"
     ;;
 
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