ci: add release workflow + install script

- Release workflow: builds linux/darwin amd64/arm64 on tag push
- Injects version via -ldflags
- Creates Gitea release with binary assets + checksums
- install.sh: curl-pipe-bash installer from latest release
- Version variable in main.go for -version flag support

cmd/review-bot/main.go

@@ -1,5 +1,6 @@
 package main
 
+
 import (
 	"flag"
 	"fmt"
@@ -13,6 +14,8 @@ import (
 	"gitea.weiker.me/rodin/review-bot/review"
 )
 
+var version = "dev"
+
 func main() {
 	// CLI flags
 	giteaURL := flag.String("gitea-url", envOrDefault("GITEA_URL", ""), "Gitea instance URL")