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/ci.yml

@@ -119,6 +119,13 @@ jobs:
           DEPLOY_KEY: ${{ secrets.INFRA_DEPLOY_KEY }}
         run: |
           set -euo pipefail
+          # infra#311: fail on an empty tag BEFORE patching anything. The
+          # grep verification further down CANNOT catch it -- with IMAGE_TAG
+          # empty its pattern matches the very corruption it exists to reject.
+          # The result is `image: <repo>:` in the manifest, which is invalid
+          # YAML, so Flux fails the ENTIRE apps Kustomization and every app
+          # stops reconciling, not just this one. That happened on 2026-08-25.
+          [ -n "${IMAGE_TAG:-}" ] || { echo "IMAGE_TAG is empty - refusing to patch"; exit 1; }
 
           # INFRA_DEPLOY_KEY is a Gitea org secret (mathias org), already
           # configured per docs/cd-pipeline.md in the infra repo — same key