feat: 新增 net pids 端口占用查询并补齐 README

- kopsc net pids <端口> 等价 lsof -ti:<端口>，输出 PID 升序去重、每行一个
- Linux 回退链 lsof -> ss -> /proc；macOS 使用 lsof；Windows 解析 netstat -ano
- 端口 1-65535 校验，无占用时无输出并正常退出
- README 新增 net.pids 小节，修正过时的匹配示例、-n 例外说明与目录结构

internal/registry/registry_test.go

@@ -25,7 +25,7 @@ func TestBuiltinIDsStable(t *testing.T) {
 		"docker.prunebuilder", "docker.prunevolumes", "docker.clearall",
 		"docker.compose.up", "docker.compose.down", "docker.compose.ps", "docker.compose.logs",
 		"diagnose",
-		"disk.usage", "net.ports", "pkg.refreshindex",
+		"disk.usage", "net.pids", "net.ports", "pkg.refreshindex",
 	}
 	if got := reg.IDs(); !reflect.DeepEqual(got, want) {
 		t.Fatalf("IDs() =\n%v\nwant\n%v", got, want)