fix(release): goreleaser v2 expects 'checksum' (singular) for sign artifacts (#19)

The first v0.2.0 tag failed with:
  invalid list of artifacts to sign: checksums

GoReleaser v2's schema enum is 'checksum' — the plural form was a v1
holdover that v2 explicitly rejects. Same Sigstore cosign keyless flow,
just the YAML key fixed.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: mastermanas805

.goreleaser.yml

@@ -101,7 +101,10 @@ checksum:
 signs:
   - id: cosign-checksums
     cmd: cosign
-    artifacts: checksums
+    # GoReleaser v2 uses the singular `checksum` (the schema's enum), not
+    # `checksums`. The earlier v1 docs floated both — the v2 release
+    # rejected the plural form with `invalid list of artifacts to sign`.
+    artifacts: checksum
     signature: "${artifact}.sig"
     certificate: "${artifact}.pem"
     args: