feat(ci): auto-redeploy ci3 dashboard on push to next (#23601)

Adds `.github/workflows/ci3-dashboard-deploy.yml` so changes under
`ci3/dashboard/**` (and the workflow itself) automatically run
`ci3/dashboard/deploy.sh` after they land on `next`. Also exposed via
`workflow_dispatch` so the deploy can be triggered manually from the
Actions tab without an SSH-capable workstation.

Today the dashboard at https://ci.aztec-labs.com is updated by an
operator running `ci3/dashboard/deploy.sh` locally, which rsyncs the
`rkapp` files to `ubuntu@ci.aztec-labs.com` and `systemctl restart
rkapp`s. That left e.g. PR #23160 (`<path:key>` route fix for
`merge-train/*` history) stuck waiting for a manual ship after merge.

Mechanics:
- Trigger: `push` to `next` with a `paths:` filter on `ci3/dashboard/**`
so only dashboard-touching merges deploy. `workflow_dispatch` for manual
runs.
- `concurrency: { group: ${{ github.workflow }}, cancel-in-progress:
false }` — at most one deploy in flight, never cancel one
mid-rsync/restart.
- Reuses the existing `BUILD_INSTANCE_SSH_KEY` secret already wired into
`avm-circuit-inputs.yml` / `claudebox.yml`, decoded to
`~/.ssh/build_instance_key` exactly the same way.
- `ssh-keyscan ci.aztec-labs.com >> ~/.ssh/known_hosts` before the
script runs so `deploy.sh` doesn't need editing — keeps the script
reusable from a workstation unchanged.

After this lands, the next merge that touches `ci3/dashboard/**` will
redeploy on its own; existing pending fixes can be shipped immediately
via the Actions tab's "Run workflow" button without that.

---
*Created by
[claudebox](https://claudebox.work/v2/sessions/295e882c4b4ae8db) ·
group: `slackbot`*

Author: spalladino

.github/workflows/ci3-dashboard-deploy.yml

@@ -0,0 +1,35 @@
+name: CI3 Dashboard Deploy
+
+on:
+  push:
+    branches:
+      - next
+    paths:
+      - "ci3/dashboard/**"
+      - ".github/workflows/ci3-dashboard-deploy.yml"
+  workflow_dispatch:
+
+concurrency:
+  group: ${{ github.workflow }}
+  cancel-in-progress: false
+
+jobs:
+  deploy:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd
+        with:
+          persist-credentials: false
+
+      - name: Deploy ci3 dashboard
+        env:
+          BUILD_INSTANCE_SSH_KEY: ${{ secrets.BUILD_INSTANCE_SSH_KEY }}
+        run: |
+          set -eu
+          mkdir -p ~/.ssh
+          echo "${BUILD_INSTANCE_SSH_KEY}" | base64 --decode > ~/.ssh/build_instance_key
+          chmod 600 ~/.ssh/build_instance_key
+          ssh-keyscan -H ci.aztec-labs.com >> ~/.ssh/known_hosts 2>/dev/null
+
+          ./ci3/dashboard/deploy.sh