ci: point storyboard runner at @adcp/sdk latest instead of adcp-3.1

The @adcp/sdk publish process changed to publish stable releases under the
`latest` dist-tag and no longer emits an `adcp-3.1` tag (AdCP 3.1 is the
current stable line, shipped as `latest`). The `adcp-3.1` tag the storyboard
jobs installed has been removed from npm, so every storyboard runner now
fails with `npm notarget @adcp/sdk@adcp-3.1`.

Track `latest` for the current-line storyboard coverage and keep the sticky
`adcp-3.0` tag for backwards-compatibility assertion.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: andybevan-scope3

.github/workflows/ci.yml

@@ -7,9 +7,10 @@ on:
     branches: [main]
 
 # Default @adcp/sdk runner alias for non-matrix storyboard jobs.
-# The reference seller storyboard job below runs both sticky 3.0 and 3.1 tags.
+# The reference seller storyboard job below runs both the sticky adcp-3.0
+# compatibility tag and the current ``latest`` stable line.
 env:
-  ADCP_SDK_VERSION: "adcp-3.1"
+  ADCP_SDK_VERSION: "latest"
 
 concurrency:
   group: ci-${{ github.ref }}
@@ -361,15 +362,15 @@ jobs:
     name: AdCP storyboard runner — examples/seller_agent.py (@adcp/sdk ${{ matrix.adcp-sdk-tag }})
     runs-on: ubuntu-latest
     # Blocking gate: examples/seller_agent.py is the Python-owned
-    # reference target for bidirectional storyboard interop. The npm
-    # dist-tags are sticky compatibility gates: ``adcp-3.0`` stays on
-    # the 3.0 runner line, while ``adcp-3.1`` tracks the current 3.1
-    # runner. Downstream SDKs and adopters can assert backwards
-    # compatibility without depending on moving latest/beta semantics.
+    # reference target for bidirectional storyboard interop. ``adcp-3.0``
+    # is a sticky compatibility tag pinned to the AdCP 3.0 runner line, so
+    # adopters can assert backwards compatibility against a fixed target;
+    # ``latest`` tracks the current stable @adcp/sdk release (AdCP 3.1 is
+    # the current stable line, published under ``latest``).
     strategy:
       fail-fast: false
       matrix:
-        adcp-sdk-tag: ["adcp-3.0", "adcp-3.1"]
+        adcp-sdk-tag: ["adcp-3.0", "latest"]
 
     steps:
       - uses: actions/checkout@v6