fix(ci): report merge train merged alerts against the base branch (#24939)

## Summary

Retargeted to `merge-train/spartan-v5` per request. The matching
`merge-train/spartan` port is
[aztec-packages#24940](#24940).

The merge-train merged Slack alert was reporting the PR head branch as
the destination. For
[aztec-packages#24851](#24851),
the PR head was `merge-train/spartan-v5` but the actual base was
`v5-next`, so the Slack message was misleading.

## Changes

- Pass `github.event.pull_request.base.ref` to the merged notification
step as `TARGET_BRANCH`.
- Make `ci3/merge_train_failure_slack_notify --merged` prefer
`TARGET_BRANCH`, with a `REF_NAME` fallback for direct/local use.

## Tests

- `bash -n ci3/merge_train_failure_slack_notify`
- `git diff --check`

---
*Created by
[claudebox](https://claudebox.work/v2/sessions/85cdf7ecc9727322/jobs/1)
· group: `slackbot` · requested by Facundo Carreiro · [Slack
thread](https://aztecprotocol.slack.com/archives/C0AU8BULZHC/p1784809391722469?thread_ts=1784809391.722469&cid=C0AU8BULZHC)*

Author: fcarreiro

.github/workflows/merge-queue-dequeue-notify.yml

@@ -37,5 +37,6 @@ jobs:
         env:
           SLACK_BOT_TOKEN: ${{ secrets.SLACK_BOT_TOKEN }}
           REF_NAME: ${{ github.event.pull_request.head.ref }}
+          TARGET_BRANCH: ${{ github.event.pull_request.base.ref }}
           PR_URL: ${{ github.event.pull_request.html_url }}
         run: ./ci3/merge_train_failure_slack_notify --merged

ci3/merge_train_failure_slack_notify

@@ -54,7 +54,8 @@ if [[ "$MODE" == "dequeued" ]]; then
     --target-ref "origin/$REF_NAME"
 
 elif [[ "$MODE" == "merged" ]]; then
-  "$TOPDIR/ci3/slack_notify" "PR merged into $REF_NAME: <$PR_URL|$commit_title> by $commit_author" "$channel"
+  merged_target="${TARGET_BRANCH:-$REF_NAME}"
+  "$TOPDIR/ci3/slack_notify" "PR merged into $merged_target: <$PR_URL|$commit_title> by $commit_author" "$channel"
 
 else
   "$TOPDIR/ci3/slack_notify_with_claudebox_kickoff" "$channel" \