chore: add release script and make target

Usage: make release VERSION=0.8.0
Bumps Cargo.toml + Cargo.lock, commits, tags, pushes — triggers
the existing GitHub Actions release workflow.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Makefile

@@ -1,4 +1,4 @@
-.PHONY: all build lint fmt check audit test coverage bench clean deploy blog
+.PHONY: all build lint fmt check audit test coverage bench clean deploy blog release
 
 all: lint build test
 
@@ -33,6 +33,12 @@ blog:
 		echo "  $$f → site/blog/posts/$$name.html"; \
 	done
 
+release:
+ifndef VERSION
+	$(error Usage: make release VERSION=0.8.0)
+endif
+	./scripts/release.sh $(VERSION)
+
 clean:
 	cargo clean