feat(cmd): add -target flag support for build commands

- Add Target flag variable to support -target parameter
- Update AddBuildFlags to include target platform option
- Enable syntax: -target platform (e.g., rp2040, wasi)

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

cmd/internal/flags/flags.go

@@ -13,11 +13,13 @@ func AddOutputFlags(fs *flag.FlagSet) {
 var Verbose bool
 var BuildEnv string
 var Tags string
+var Target string
 
 func AddBuildFlags(fs *flag.FlagSet) {
 	fs.BoolVar(&Verbose, "v", false, "Verbose mode")
 	fs.StringVar(&Tags, "tags", "", "Build tags")
 	fs.StringVar(&BuildEnv, "buildenv", "", "Build environment")
+	fs.StringVar(&Target, "target", "", "Target platform (e.g., rp2040, wasi)")
 }
 
 var Gen bool