revert: 恢复版本号动态注入，本地构建回退为 dev

本地运行不再硬编码 0.3.3，改回 dev；build.sh 通过 git describe
注入真实 tag 并经 ldflags 覆盖 main.version。

Claude-Session: https://claude.ai/code/session_01HMRoSjvobCcuH5gbKkStFF

cmd/artex/main.go

@@ -21,9 +21,9 @@ import (
 	"github.com/Autumn-27/artex/server"
 )
 
-// version is the application version. Release builds may override it via
-// -ldflags "-X main.version=<version>".
-var version = "0.3.3"
+// version is the build version, injected at release time via
+// -ldflags "-X main.version=<tag>". Defaults to "dev" for local builds.
+var version = "dev"
 
 const banner = `
     _    ____ _____ _______  __