agentcli: Windows ConPTY 适配分支（原生实现替代剔除桩）+ cabi loader.c build tag 修复

- agentcli 平台无关化：plugin.go 拆为公共层 + ptyTerm 接口，新增
  pty_linux.go（/dev/ptmx 原实现搬移，行为不变）与 pty_windows.go
  （ConPTY 原生实现，CreatePseudoConsole + PROC_THREAD_ATTRIBUTE_PSEUDOCONSOLE
  注入子进程，双管道读写/ResizePseudoConsole/GetExitCodeProcess，零第三方依赖）
- plugin_stub.go 收紧为 !linux && !windows（darwin 等仍走桩）
- plugin_test.go build tag 放宽到 linux || windows
- cabi/loader.c 补 linux || darwin build tag，修复 Windows 下
  cgo 禁用时残留 C 源文件的编译错误
- 验证：Linux 全量构建+测试通过；Windows CGO 交叉编译 + go vet 全包零错误

internal/plugins/agentcli/pty_linux.go

@@ -0,0 +1,133 @@
+//go:build linux
+
+package agentcli
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"syscall"
+	"unsafe"
+)
+
+// PTY ioctl constants for Linux
+const (
+	TIOCGPTN   = 0x80045430
+	TIOCSPTLCK = 0x40045431
+	TIOCSWINSZ = 0x5414
+)
+
+type winsize struct {
+	Row    uint16
+	Col    uint16
+	XPixel uint16
+	YPixel uint16
+}
+
+func ioctl(fd, cmd uintptr, ptr unsafe.Pointer) error {
+	_, _, errno := syscall.Syscall(syscall.SYS_IOCTL, fd, cmd, uintptr(ptr))
+	if errno != 0 {
+		return errno
+	}
+	return nil
+}
+
+func openPty() (master *os.File, slave *os.File, err error) {
+	mfd, err := syscall.Open("/dev/ptmx", syscall.O_RDWR|syscall.O_NOCTTY, 0)
+	if err != nil {
+		return nil, nil, fmt.Errorf("open /dev/ptmx: %w", err)
+	}
+	master = os.NewFile(uintptr(mfd), "/dev/ptmx")
+
+	var unlock int32
+	if err := ioctl(uintptr(mfd), TIOCSPTLCK, unsafe.Pointer(&unlock)); err != nil {
+		master.Close()
+		return nil, nil, fmt.Errorf("TIOCSPTLCK: %w", err)
+	}
+
+	var ptyno int32
+	if err := ioctl(uintptr(mfd), TIOCGPTN, unsafe.Pointer(&ptyno)); err != nil {
+		master.Close()
+		return nil, nil, fmt.Errorf("TIOCGPTN: %w", err)
+	}
+
+	slavePath := fmt.Sprintf("/dev/pts/%d", ptyno)
+	sfd, err := syscall.Open(slavePath, syscall.O_RDWR|syscall.O_NOCTTY, 0)
+	if err != nil {
+		master.Close()
+		return nil, nil, fmt.Errorf("open slave %s: %w", slavePath, err)
+	}
+	slave = os.NewFile(uintptr(sfd), slavePath)
+
+	return master, slave, nil
+}
+
+func defaultShell() string { return "bash" }
+
+// linuxPty 基于 Linux PTY 的终端后端。
+type linuxPty struct {
+	master *os.File
+	slave  *os.File
+	cmd    *exec.Cmd
+}
+
+func (p *linuxPty) Read(buf []byte) (int, error) { return p.master.Read(buf) }
+
+func (p *linuxPty) WriteString(s string) (int, error) { return p.master.WriteString(s) }
+
+func (p *linuxPty) Resize(rows, cols uint16) error {
+	ws := winsize{Row: rows, Col: cols}
+	if err := ioctl(uintptr(p.master.Fd()), TIOCSWINSZ, unsafe.Pointer(&ws)); err != nil {
+		return fmt.Errorf("TIOCSWINSZ: %w", err)
+	}
+	return nil
+}
+
+// Running 在 Linux 上保持旧语义：进程退出通过 master EOF 由 readLoop/cleanup 感知，
+// 因此这里恒返回 true，行为与改造前一致。
+func (p *linuxPty) Running() bool { return true }
+
+func (p *linuxPty) Kill() error {
+	if p.cmd != nil && p.cmd.Process != nil {
+		return p.cmd.Process.Kill()
+	}
+	return nil
+}
+
+func (p *linuxPty) Close() error {
+	p.slave.Close()
+	return p.master.Close()
+}
+
+// newCommandPty 创建 PTY 并在其上启动子命令（sh -c）。
+func newCommandPty(command string, rows, cols uint16) (ptyTerm, *exec.Cmd, error) {
+	master, slave, err := openPty()
+	if err != nil {
+		return nil, nil, err
+	}
+
+	cmd := exec.Command("sh", "-c", command)
+	cmd.Stdin = slave
+	cmd.Stdout = slave
+	cmd.Stderr = slave
+	cmd.SysProcAttr = &syscall.SysProcAttr{
+		Setsid:  true,
+		Setctty: true,
+		Ctty:    0,
+	}
+
+	if err := cmd.Start(); err != nil {
+		master.Close()
+		slave.Close()
+		return nil, nil, fmt.Errorf("start command: %w", err)
+	}
+
+	slave.Close()
+	pt := &linuxPty{master: master, cmd: cmd}
+	if err := pt.Resize(rows, cols); err != nil {
+		master.Close()
+		cmd.Process.Kill()
+		return nil, nil, fmt.Errorf("resize pty: %w", err)
+	}
+	return pt, cmd, nil
+}