fix: 拆分 starssh 的拨号超时与认证超时语义

- 为 LoginInput 新增 DialTimeout，明确区分【TCP/proxy/ssh-agent 拨号超时】和【SSH 握手/认证超时】
- 将 Timeout 收口为握手/认证阶段超时，0 表示不限制，不再在登录入口自动回填默认值
- 新增 effectiveLoginTimeout/effectiveDialTimeout，统一超时决策逻辑
- 调整 login 流程，仅对 login context、ssh.ClientConfig 和握手阶段连接 deadline 使用认证超时
- 调整 transport 拨号链路，默认 TCP dial、proxy dial 与 ssh-agent 建连统一改用 DialTimeout
- 修正 agent forwarding 初始化仍错误复用 LoginInfo.Timeout 的问题
- 保持 LoginSimple 的直观行为：传入 timeout 时同时映射到 Timeout 和 DialTimeout
- 新增 login_timeout_test，覆盖零值不回填、DialTimeout 优先级，以及 ssh-agent 认证路径使用拨号超时的回归测试

types.go

@@ -94,14 +94,20 @@ type LoginInput struct {
 	AuthOrder                   []AuthMethodKind
 	Addr                        string
 	Port                        int
-	Timeout                     time.Duration
-	DialContext                 DialContextFunc
-	Proxy                       *ProxyConfig
-	Jump                        *LoginInput
-	KeepAliveInterval           time.Duration
-	KeepAliveTimeout            time.Duration
-	HostKeyCallback             func(string, net.Addr, ssh.PublicKey) error
-	BannerCallback              func(string) error
+	// Timeout limits the SSH handshake/authentication phase after a TCP connection has
+	// already been established. Zero means no authentication timeout.
+	Timeout time.Duration
+	// DialTimeout limits outbound dial steps such as TCP connect, proxy connect, and
+	// local ssh-agent socket connect. Zero falls back to Timeout when set, otherwise
+	// uses the package default dial timeout. Negative disables the default dial timeout.
+	DialTimeout       time.Duration
+	DialContext       DialContextFunc
+	Proxy             *ProxyConfig
+	Jump              *LoginInput
+	KeepAliveInterval time.Duration
+	KeepAliveTimeout  time.Duration
+	HostKeyCallback   func(string, net.Addr, ssh.PublicKey) error
+	BannerCallback    func(string) error
 }
 
 // StarShell keeps the legacy prompt-driven helper for POSIX-style scripted shell interactions.