dharma: pin deployment image to SHA tag (placeholder)

Replaces `:latest` + `imagePullPolicy: Always` with a content-
addressable SHA tag produced by the dharma-el ci-prod workflow. The
deployed image is now deterministic and rollback is `git revert` of
this file.

`PINNED_BY_NEXT_BUILD` is a deliberate placeholder — Argo CD will fail
to reconcile (ImagePullBackOff) until a human replaces it with a real
short SHA. That failure is the forcing function: prod only gets a new
image via a PR that names a real, built tag.

This PR is opened as a draft. Do not merge until:
  1. dharma-el#1 (feature/ci-prod-image-build) is merged
  2. Dharma CI — prod runs on main and produces a SHA-tagged image
  3. The placeholder here is replaced with that SHA

`imagePullPolicy: Always` is left in place for now — redundant once
we're on SHA tags but removing it in the same PR adds risk. Drop it
in a follow-up.

servers/legion/k8s/neuron-technologies/dharma/deployment.yaml

@@ -25,8 +25,19 @@ spec:
           type: RuntimeDefault
       containers:
         - name: dharma
-          image: registry.neuralplatform.ai/neuron-technologies/dharma:latest
-          imagePullPolicy: Always
+          # Pinned to a content-addressable SHA tag instead of :latest so the
+          # deployed image is deterministic and rollback is `git revert`. The
+          # tag is produced by the dharma-el ci-prod.yaml workflow on every
+          # push to main: registry.neuralplatform.ai/neuron-technologies/
+          # dharma:<short-sha>.
+          #
+          # PINNED_BY_NEXT_BUILD is a deliberate placeholder. It will fail to
+          # pull (ImagePullBackOff) until a human replaces it with a real
+          # short SHA from a successful ci-prod run. That failure is the
+          # forcing function: the only way prod gets a new image is by
+          # opening a PR that names a real, built tag.
+          image: registry.neuralplatform.ai/neuron-technologies/dharma:PINNED_BY_NEXT_BUILD
+          imagePullPolicy: Always  # redundant once on SHA tags; remove in a follow-up
           ports:
             - name: http
               containerPort: 8765