feat: native SSH delegation via tea.ExecProcess + Gruvbox palette

Phase A — Tamagosh-inspired redesign:
- Replace goroutine-based SSH (crypto/ssh) with tea.ExecProcess + native ssh binary
- Zero-lag SSH: native binary pegang TTY langsung, ga ada Go buffering
- Password auth via sshpass -e (SSHPASS env)
- Key auth via ssh -i <tmpfile> + SSH_ASKPASS for passphrase
- ControlMaster sockets for fast reconnect
- Add askpass subcommand for SSH_ASKPASS support
- Gruvbox Material Dark Hard palette for comfortable viewing
- Clean up old session messages (openSessionMsg, sessionOutputMsg, sessionDoneMsg)
- Remove 354-line session.go, replace with 150-line ssh.go

pkg/tui/host_list_tab.go

@@ -58,7 +58,7 @@ func (t *HostListTab) Update(msg tea.Msg) (Tab, tea.Cmd) {
 			if len(t.hosts) > 0 {
 				host := t.hosts[t.selectedIndex]
 				return t, func() tea.Msg {
-					return openSessionMsg{host: host}
+					return sshConnectToMsg{host: host}
 				}
 			}