chore(ci): route spartan/spartan-v5 merge-train notifications to #team-alpha-ci (#24963)

## What

Two related changes for the spartan / spartan-v5 merge-train Slack
notifications:

1. **Route them to `#team-alpha-ci`** instead of `#team-alpha`, so the
team channel stops receiving CI noise.
2. **Stop silently dropping failed Slack posts** in `ci3/slack_notify`,
so a mis-delivered notification (e.g. the bot isn't a member of the
target channel) is visible in the CI log instead of vanishing.

## (1) Notification sites re-routed

| File | Notification | Before → After |
|---|---|---|
| `ci3/merge_train_failure_slack_notify` | "PR merged into …", CI
failure, and dequeue notifications for spartan / spartan-v5 (each also
kicks off a ClaudeBox investigation) | `#team-alpha` → `#team-alpha-ci`
|
| `.github/workflows/merge-train-stale-check.yml` | daily
"`merge-train/spartan…` has not merged into `next`/`v5-next`" stale
alert | `#team-alpha` → `#team-alpha-ci` |
| `.github/workflows/network-healthcheck.yml` | "Starting network
healthcheck …" kickoff + the ClaudeBox `respond_to_user` summary |
`#team-alpha` → `#team-alpha-ci` |
| `scripts/socket-fix-ci.sh` | Socket dependency-vulnerability fix
notification | `#team-alpha` → `#team-alpha-ci` |
| `ci3/merge_train_stale_check` | usage-example comment only | updated
for consistency |

`ci3/run_test_cmd` — flake notifications for spartan / spartan-v5
**already** post to `#team-alpha-ci` (since
[#23219](#23219),
2026-05-12).

## (2) Surface Slack post failures (`ci3/slack_notify`)

Previously the `chat.postMessage` curl ended in `&>/dev/null`, so any
failure — most commonly `not_in_channel` when the bot isn't a member of
the target channel — disappeared with no trace. This is the likely
reason flake notifications to `#team-alpha-ci` were never observed
despite the routing being correct: the posts were failing silently. The
script now captures the API response and logs a clear error to stderr
(`slack_notify: failed to post to Slack channel '<chan>': <error>`) on
any non-`ok` response or curl/network failure. It stays best-effort —
the failure is logged, the caller is not failed.

## Base & backport

Based on **`merge-train/spartan`** (targets `next`), and labeled
**`backport-to-v5-next`** so the same change is carried over to the
`v5-next` line. The `network-healthcheck` and `merge-train-stale-check`
schedules ultimately run from the default branch once this reaches
`next` via the train; the per-train CI scripts
(`merge_train_failure_slack_notify`, `slack_notify`, `socket-fix-ci.sh`)
run from the checked-out train head.

## Left untouched (intentionally)

- `.claude/skills/merge-trains/SKILL.md` team-mapping table,
`ci3/dashboard/.../ci-health-report.html`, `spartan/testnet-runbook.md`
— these reference the owning **team** (`#team-alpha`) or are a manual
human runbook step, not automated CI notifications.
- Other merge-train channel mappings (`#team-bonobos`, `#honk-team`,
`#help-ci`, `#dev-rels`, `#team-fairies`) are unrelated and unchanged.

## Notes

- No new Slack scopes are required — every call uses the same
`SLACK_BOT_TOKEN` + `chat.postMessage`. The bot must be a **member of
`#team-alpha-ci`** for delivery; once (2) lands, a missing membership
shows up as a `not_in_channel` line in the CI log. The **ClaudeBox app**
must also be a member so the healthcheck summary and failure/dequeue
kickoff replies can post.

---
*Created by
[claudebox](https://claudebox.work/v2/sessions/e345f34e9b2da947/jobs/1)
· group: `slackbot` · requested by Santiago Palladino · [Slack
thread](https://aztecprotocol.slack.com/archives/C0AU8BULZHC/p1784902660161109?thread_ts=1784902660.161109&cid=C0AU8BULZHC)*

Author: AztecBot

.github/workflows/merge-train-stale-check.yml

@@ -20,7 +20,7 @@ jobs:
         env:
           GH_TOKEN: ${{ secrets.AZTEC_BOT_GITHUB_TOKEN }}
           SLACK_BOT_TOKEN: ${{ secrets.SLACK_BOT_TOKEN }}
-        run: ./ci3/merge_train_stale_check merge-train/spartan '#team-alpha'
+        run: ./ci3/merge_train_stale_check merge-train/spartan '#team-alpha-ci'
 
   spartan-v5:
     name: Check merge-train/spartan-v5
@@ -36,7 +36,7 @@ jobs:
           GH_TOKEN: ${{ secrets.AZTEC_BOT_GITHUB_TOKEN }}
           SLACK_BOT_TOKEN: ${{ secrets.SLACK_BOT_TOKEN }}
           BASE_BRANCH: v5-next
-        run: ./ci3/merge_train_stale_check merge-train/spartan-v5 '#team-alpha'
+        run: ./ci3/merge_train_stale_check merge-train/spartan-v5 '#team-alpha-ci'
 
   fairies-v5:
     name: Check merge-train/fairies-v5

.github/workflows/network-healthcheck.yml

@@ -37,14 +37,14 @@ jobs:
           5. Any errors (level >= 50) or warnings (level 40) in the last 8 hours
           6. Bot status if applicable
 
-          Create a gist with the full healthcheck report. Then post a concise summary to the #team-alpha channel via respond_to_user. Flag anything that needs attention (stopped bots, missed slots, errors, low peer counts).
+          Create a gist with the full healthcheck report. Then post a concise summary to the #team-alpha-ci channel via respond_to_user. Flag anything that needs attention (stopped bots, missed slots, errors, low peer counts).
 
           Format the respond_to_user message as a brief network status overview, e.g.:
           - testnet: healthy, block 5570, 100 peers
           - mainnet: healthy, block 1234, 50 peers
           - devnet: WARNING - bot stopped (insufficient balance)
           Link to the gist for full details."
 
-          ./ci3/slack_notify_with_claudebox_kickoff "#team-alpha" \
+          ./ci3/slack_notify_with_claudebox_kickoff "#team-alpha-ci" \
             "Starting network healthcheck for: ${NETWORKS}" \
             "$PROMPT"

ci3/merge_train_failure_slack_notify

@@ -36,9 +36,9 @@ elif [[ "$REF_NAME" == "merge-train/fairies" ]]; then
 elif [[ "$REF_NAME" == "merge-train/fairies-v5" ]]; then
   channel="#team-fairies"
 elif [[ "$REF_NAME" == "merge-train/spartan" ]]; then
-  channel="#team-alpha"
+  channel="#team-alpha-ci"
 elif [[ "$REF_NAME" == "merge-train/spartan-v5" ]]; then
-  channel="#team-alpha"
+  channel="#team-alpha-ci"
 else
   exit 0
 fi

ci3/merge_train_stale_check

@@ -6,7 +6,7 @@
 # Usage: merge_train_stale_check <merge-train-branch> <slack-channel>
 #
 # Example:
-#   merge_train_stale_check merge-train/spartan '#team-alpha'
+#   merge_train_stale_check merge-train/spartan '#team-alpha-ci'
 #
 # Required env vars:
 #   GH_TOKEN        — GitHub API token (used by `gh api`)

ci3/slack_notify

@@ -8,7 +8,17 @@ fi
 channel=${2:-"#aztec3-ci"}
 data=$(jq -n --arg channel "$channel" --arg text "$1" '{channel: $channel, text: $text}')
 
-curl -X POST https://slack.com/api/chat.postMessage \
+# Post to Slack, capturing the API response. Previously the response went to
+# /dev/null, so a failed post (most commonly the bot not being a member of the
+# target channel -> "not_in_channel", or an invalid channel/token) vanished with
+# no trace and could not be troubleshooted. Surface any failure in the CI log.
+# Best-effort: log the failure but do not fail the caller.
+response=$(curl -sS -X POST https://slack.com/api/chat.postMessage \
   -H "Authorization: Bearer $SLACK_BOT_TOKEN" \
   -H "Content-type: application/json" \
-  --data "$data" &>/dev/null
+  --data "$data" 2>&1)
+
+if [ "$(jq -r '.ok // false' <<<"$response" 2>/dev/null)" != "true" ]; then
+  error=$(jq -r '.error // empty' <<<"$response" 2>/dev/null)
+  echo "slack_notify: failed to post to Slack channel '$channel': ${error:-$response}" >&2
+fi

scripts/socket-fix-ci.sh

@@ -122,7 +122,7 @@ ${LINKS}"
 ${LINKS}"
     fi
 
-    data=$(jq -n --arg channel "#team-alpha" --arg text "$TEXT" '{channel: $channel, text: $text}')
+    data=$(jq -n --arg channel "#team-alpha-ci" --arg text "$TEXT" '{channel: $channel, text: $text}')
 
     curl -X POST https://slack.com/api/chat.postMessage \
       -H "Authorization: Bearer $SLACK_BOT_TOKEN" \