ci: guard crates.io publish when the token is absent

Skip cargo publish (with a warning) if CARGO_REGISTRY_TOKEN is unset.

Author: ajsb85

.github/workflows/publish-crate.yml

@@ -13,6 +13,8 @@ jobs:
   publish:
     name: Publish to crates.io
     runs-on: ubuntu-latest
+    env:
+      CARGO_REGISTRY_TOKEN: ${{ secrets.CARGO_REGISTRY_TOKEN }}
     steps:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@stable
@@ -21,7 +23,9 @@ jobs:
           cargo fmt --check
           cargo clippy --all-targets --all-features -- -D warnings
           cargo test --all-features
-      - name: Publish
-        env:
-          CARGO_REGISTRY_TOKEN: ${{ secrets.CARGO_REGISTRY_TOKEN }}
+      - name: Publish to crates.io
+        if: env.CARGO_REGISTRY_TOKEN != ''
         run: cargo publish
+      - name: Skip publish (no token configured)
+        if: env.CARGO_REGISTRY_TOKEN == ''
+        run: echo "::warning::CARGO_REGISTRY_TOKEN secret is not set; skipping crates.io publish. Add it under repo Settings > Secrets and re-run."