ci: keep ably/features reusable workflow tracking main

Revert the SHA pin on `ably/features/.github/workflows/sdk-features.yml`
back to `@main`. This workflow is the SDK feature-spec compliance
suite — pinning it freezes the spec under test, which defeats the
purpose of running it on every PR. The repo is org-owned and trusted,
so the `unpinned-uses` finding is suppressed inline rather than
fixed.

Author: emptyhammond

.github/workflows/features.yml

@@ -11,7 +11,8 @@ permissions:
 
 jobs:
   build:
-    uses: ably/features/.github/workflows/sdk-features.yml@6b3fc7a8ede2ebdd7a6325314f3a96c6466f1453 # main as of 2026-05-26
+    # Track the live SDK feature spec on main rather than pinning.
+    uses: ably/features/.github/workflows/sdk-features.yml@main # zizmor: ignore[unpinned-uses]
     with:
       repository-name: ably-dotnet
     secrets: