ci: add goreleaser and semver-driven release workflow

The workflow calculates the version with semver-generator, tags the
commit when the version is new, and runs goreleaser. goreleaser ships
two distribution paths: tar.gz binaries for darwin and linux on amd64
and arm64, and a multi-arch ghcr.io container that carries the go,
python3, and C toolchains so the exec evaluators run in-container.
The new llmtest version subcommand reports the stamped build version.

semver.yaml

@@ -0,0 +1,20 @@
+version: 1
+force:
+  existing: true
+blacklist:
+  - "Merge branch"
+  - "Merge pull request"
+wording:
+  patch:
+    - fix
+    - chore
+    - docs
+    - style
+    - refactor
+    - perf
+    - ci
+  minor:
+    - feat
+  major:
+    - "breaking change"
+    - "major overhaul"