feat: GitFlow workflow with beta/stable CI pipelines

- Add develop branch for integration
- Replace single ci.yml with 3 workflows: ci-pr (PR checks), ci-develop (beta releases), ci-main (stable releases)
- Add prerelease support in version.go (ldflags injection)
- Add tests for prerelease version formatting
- Document full GitFlow process, versioning, and contributing guide in README

💘 Generated with Crush

Assisted-by: GLM-5.1 via Crush <crush@charm.land>

internal/version/version.go

@@ -7,6 +7,12 @@ const (
 	License = "MIT"
 )
 
+var Prerelease string
+
 func FullVersion() string {
-	return Name + " v" + Version
+	v := Name + " v" + Version
+	if Prerelease != "" {
+		v += "-" + Prerelease
+	}
+	return v
 }