docker: address review follow-ups (pip-shim flags, sync ownership, tags, zoo ref, arch list)

- pip shim: do not treat the value of an index-url / find-links / constraint flag
  as an install target. A cell like 'pip install --extra-index-url <url> torch'
  now no-ops after keeping the baked stack instead of exec'ing a bare
  'pip install --extra-index-url <url>' that fails. Positional . / url / vcs and
  -r/--requirement files still count as targets.
- notebook sync: on first boot, record only files we actually wrote (or that are
  byte-identical to the template), never a kept pre-existing user file; and on the
  GitHub refresh, treat a file present in DEST but absent from the sync state as
  user-owned and keep it. Previously a bind-mounted notebook was recorded as
  managed and then overwritten by upstream.
- docker-publish: add flavor latest=false to the Studio metadata steps too, so a
  v* tag push cannot emit an implicit :latest via metadata-action's latest=auto;
  :latest stays default-branch-only, and the smoke test pulls the published tag.
- unsloth-studio-update: resolve the unsloth-zoo ref independently of --ref (new
  --zoo-ref, else use the ref only when the zoo repo has it, else fall back to
  main) so 'update --ref <unsloth-tag/sha>' does not fail on a missing zoo ref.
- Dockerfile: drop 10.3 (compute_103) from TORCH_CUDA_ARCH_LIST in both the
  builder and runtime stages. B300 runs sm_100 SASS, and the bundled CUDA 12.8
  nvcc cannot compile compute_103 (added in 12.9), which broke arch-list-honoring
  source / JIT builds.

.github/workflows/docker-publish.yml

@@ -429,9 +429,14 @@ jobs:
         uses: docker/metadata-action@v5
         with:
           images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          # latest=false disables metadata-action's implicit latest=auto, which
+          # would otherwise emit :latest on a v* tag push and bypass the
+          # default-branch-only gate below. :latest is published only by the
+          # explicit type=raw rule (default-branch pushes), matching the base job.
+          flavor: latest=false
           tags: |
             # The full Studio image owns the unprefixed namespace, headed by
-            # :latest. Same :latest gating rationale as the base job.
+            # :latest (default branch only). Tag pushes publish the version tag.
             type=raw,value=latest,enable=${{ github.ref == format('refs/heads/{0}', github.event.repository.default_branch) && github.event.inputs.unsloth_ref == '' }}
             type=ref,event=tag
             type=schedule,pattern=nightly
@@ -508,6 +513,9 @@ jobs:
         uses: docker/metadata-action@v5
         with:
           images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          # Mirror the studio tag rules (incl. latest=false) so the smoke test
+          # pulls the tag just published, not an implicit latest=auto :latest.
+          flavor: latest=false
           tags: |
             type=raw,value=latest,enable=${{ github.ref == format('refs/heads/{0}', github.event.repository.default_branch) && github.event.inputs.unsloth_ref == '' }}
             type=ref,event=tag