feat: Task 16 — SSH Session Tab (multi-session)

- pkg/tui/session.go — SessionTab with live SSH terminal in a tab
- pkg/tui/messages.go — quitMsg, openSessionMsg, sessionOutputMsg
- pkg/tui/tabs.go — Tab.Close() interface, Add() returns tea.Cmd
- pkg/tui/host_list_tab.go — Enter opens session tab, q quits
- pkg/tui/tui.go — handle openSessionMsg/quitMsg, SetDataDir
- cmd/hostkeeper/tui.go — pass dataDir to model

pkg/tui/messages.go

@@ -0,0 +1,20 @@
+package tui
+
+import "git.tukangketik.id/swanadiva/hostkeeper/internal/models"
+
+// quitMsg signals the TUI to exit
+type quitMsg struct{}
+
+// openSessionMsg signals the TUI to open a new SSH session tab
+type openSessionMsg struct {
+	host    *models.Host
+	dataDir string
+}
+
+// sessionOutputMsg carries SSH output to the TUI renderer
+type sessionOutputMsg string
+
+// sessionDoneMsg signals that a session has ended
+type sessionDoneMsg struct {
+	err error
+}