feat(cmd): enable -target parameter for build, run, and test commands

- Update build command: llgo build -target platform
- Update run command: llgo run -target platform
- Update test command: llgo test -target platform
- Wire target flag to build configuration
- Update usage documentation for new parameter

Examples:
- llgo build -target rp2040 ./firmware
- llgo run -target wasi ./main.go
- llgo test -target cortex-m ./tests

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

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

cmd/internal/build/build.go

@@ -29,7 +29,7 @@ import (
 
 // llgo build
 var Cmd = &base.Command{
-	UsageLine: "llgo build [-o output] [build flags] [packages]",
+	UsageLine: "llgo build [-o output] [-target platform] [build flags] [packages]",
 	Short:     "Compile packages and dependencies",
 }
 
@@ -50,6 +50,7 @@ func runCmd(cmd *base.Command, args []string) {
 	conf.Tags = flags.Tags
 	conf.Verbose = flags.Verbose
 	conf.OutFile = flags.OutputFile
+	conf.Target = flags.Target
 
 	args = cmd.Flag.Args()