chore(ci): route backport-CI-failure Slack alert to #backports by channel ID

[AztecBot]

Summary

Proposal to switch the "CI3 failed on backport PR" Slack notification (.github/workflows/ci3.yml, line 144) from posting by channel name (#backports) to posting by channel ID (C0AGN2WT3CP). Channel IDs are immune to renames and don't depend on Slack name resolution.

Why this is in .github-new/

This session was launched without the ci-allow flag, so direct edits under .github/ are blocked. The proposed file is at .github-new/workflows/ci3.yml. A maintainer with ci-allow should copy it over .github/workflows/ci3.yml (single-line diff below) and merge.

Diff (one line)

-              --data "$(jq -n --arg c "#backports" --arg t "$TEXT" '{channel:$c, text:$t}')"
+              --data "$(jq -n --arg c "C0AGN2WT3CP" --arg t "$TEXT" '{channel:$c, text:$t}')"

Context

The example notification:

CI3 failed on backport PR: #22924 - chore: Accumulated backports to v4-next

is emitted by ci3.yml lines 130–145 whenever a backport-to-*-staging PR fails CI. Routing by ID guarantees it reaches the backports channel.

Note for follow-up

.github/workflows/backport.yml:112 (the initial cherry-pick conflict notification) uses the same --arg c "#backports" pattern. Left untouched here per the scoped ask, but worth flipping to the channel ID for consistency in a follow-up.

Test plan

• After a maintainer copies the file into .github/, trigger a backport PR CI failure (head ref starts with backport-to- and ends with -staging) and confirm the Slack message lands in #backports.

ClaudeBox log: https://claudebox.work/s/3ee68bf1f6c07856?run=1

[alexghr]

Closing in favour of #22970