Bump opencode 1.14.44 -> 1.14.50; cut over to split-base pipeline

- Bump OPENCODE_VERSION 1.14.44 -> 1.14.50 in Dockerfile.variant
- Cut over: docker-publish-split.yml now triggers on push: tags: v*
  (was workflow_dispatch only). RELEASE_TAG and PROMOTE_LATEST derived
  from github.ref_type/ref_name for tag-push; inputs still available
  for manual workflow_dispatch runs.
- Delete docker-publish.yml (retired, replaced by split-base pipeline)
- Delete Dockerfile (retired, replaced by Dockerfile.base + Dockerfile.variant)
- Update CHANGELOG: promote Unreleased -> v1.14.50
- Update AGENTS.md, .gitea/README.md, validate.yml: remove all references
  to the old single-Dockerfile pipeline and WIP migration plan

.gitea/workflows/validate.yml

@@ -2,8 +2,8 @@ name: Validate
 
 # Lightweight validation on pushes to main. Builds single-arch (amd64),
 # runs the smoke test, and checks image size — without pushing anything
-# to Docker Hub. Tag pushes are handled by docker-publish.yml which
-# does the full multi-arch build-and-push.
+# to Docker Hub. Tag pushes are handled by docker-publish-split.yml which
+# does the full multi-arch split-base build-and-push.
 
 on:
   push: