ci(deploy): refuse to patch a manifest with an empty IMAGE_TAG (infra#311)

On 2026-08-25 a deploy job ran with an empty IMAGE_TAG, wrote
`image: localhost:5000/gitea-mcp:` into the infra repo and pushed it.
That is invalid YAML, so Flux failed the ENTIRE apps Kustomization --
every app stopped reconciling for about an hour, including three
unrelated deploys queued behind it.

This job's existing check could not catch it. It greps for
`image: <repo>:${IMAGE_TAG}` after the sed, and with IMAGE_TAG empty that
pattern matches the corrupted line itself, so the verification passes
because of the bug rather than despite it.

Asserting the input up front is the fix: an empty tag now fails the job
before anything is written, let alone committed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011PL5XcG68SvQqz3vGVbWbf

.gitea/workflows/cd.yml

@@ -45,6 +45,12 @@ jobs:
           set -e
           trap 'rm -rf /tmp/infra-update; rm -f ~/.ssh/infra_deploy_key' EXIT
           IMAGE_TAG="${{ github.sha }}"
+          # infra#311: github.sha is never empty in practice, but the manifest
+          # patch below is the same shape that took Flux down estate-wide on
+          # 2026-08-25 -- an empty tag yields `image: <repo>:`, which is invalid
+          # YAML and fails the ENTIRE Kustomization, not just this app. The
+          # assert costs nothing and removes the class.
+          [ -n "${IMAGE_TAG:-}" ] || { echo "IMAGE_TAG is empty - refusing to patch"; exit 1; }
           mkdir -p ~/.ssh
           echo "${{ secrets.INFRA_DEPLOY_KEY }}" > ~/.ssh/infra_deploy_key
           chmod 600 ~/.ssh/infra_deploy_key