fix(gitops): stop Flux ImageUpdateAutomation from clobbering ai-assistant's digest pin

The $imagepolicy marker was still present on this line, so FluxCD Image
Automation kept overwriting CI's digest-pinned image every ~2 minutes
with whatever it considered alphabetically "highest" (":latest" beats
any hex SHA tag under an ascending-alphabetical policy) - the exact
race the ai-assistant build.yaml's own comments describe having fixed,
but the marker was never actually removed for this app specifically.
admin-solid and frontend-solid don't carry this marker, which is why
their CI-driven digest pins hold; bring ai-assistant in line with that
convention so its build.yaml "Update GitOps" step is the sole writer
going forward.

Pinned to the digest currently running in the cluster
(sha256:106d0403...) after a manual rollout restart picked up
today's security-fix build.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/nxtgauge-ai-assistant/overlays/prod/release-patch.yaml

@@ -8,4 +8,4 @@ spec:
     spec:
       containers:
         - name: ai-assistant
-          image: ci.nxtgauge.com/ashwin/nxtgauge-ai-assistant:latest # {"$imagepolicy": "flux-system:nxtgauge-ai-assistant"}
+          image: ci.nxtgauge.com/ashwin/nxtgauge-ai-assistant@sha256:106d04039e0e006d3356ace26e9f776449ab9784f979ed5f17e1f36b9b742b2b