fix(release): prerelease auto-detect; changelog excludes scoped conventional commits

Two polish issues surfaced by the v0.3.1-rc1 pipeline test:

- The release was tagged v0.3.1-rc1 but published without the
  prerelease flag, so it appeared alongside stable releases. Add
  'prerelease: auto' to release.github so GoReleaser marks any tag
  with a semver prerelease suffix (-rc, -beta, -alpha, -pre)
  appropriately.

- The changelog filters used '^docs:' patterns that only match bare
  conventional commits. Scoped variants like 'docs(readme):' and
  'chore(make):' slipped through into the published changelog.
  Switch to '^docs[:(]' style patterns to match both forms, and add
  '^style[:(]' so gofmt-drift commits are excluded too.

.goreleaser.yml

@@ -37,9 +37,12 @@ changelog:
   sort: asc
   filters:
     exclude:
-      - "^docs:"
-      - "^test:"
-      - "^chore:"
+      # Match both bare and scoped conventional commits, e.g. both
+      # "docs:" and "docs(readme):" should be excluded.
+      - "^docs[:(]"
+      - "^test[:(]"
+      - "^chore[:(]"
+      - "^style[:(]"
 
 # Multi-arch Docker images published to GitHub Container Registry.
 # Build host needs Docker buildx and a `docker login ghcr.io` for the
@@ -98,3 +101,6 @@ release:
   github:
     owner: VikingOwl91
     name: gnoma
+  # Auto-detect prereleases from semver: tags with -rc, -beta, -alpha,
+  # -pre, etc. suffix get marked as prerelease on GitHub.
+  prerelease: auto