fix(ssh): restore exitAlternateScreenBuffer to prevent character loss on session open

[rlagentflowbeta]

Summary

• Restores `exitAlternateScreenBuffer()` to `releaseTerminal()` in `InteractiveSpawn` so SSH runs in the normal screen buffer, isolated from Ink's rendering activity
• Removes the `setRawMode(false)` call that was invoking `tcsetattr(TCSAFLUSH)` and switching the terminal to canonical mode during SSH connection setup

Root Cause

Two related issues were introduced when `exitAlternateScreenBuffer()` was removed from the spawn flow (in a previous refactor aimed at eliminating the brief visual flash on SSH exit):

1. SSH ran inside Ink's alternate screen buffer. Any Ink re-renders during the SSH connection setup wrote ANSI escape sequences to the same buffer SSH was using, corrupting terminal state and causing initial keystrokes to be swallowed.

2. `setRawMode(false)` was called before spawning SSH. Node.js's `setRawMode` uses `tcsetattr(TCSAFLUSH)` internally. `TCSAFLUSH` discards all pending kernel input and switches the terminal to canonical (line-buffered) mode. Characters typed during the SSH connection handshake were therefore either discarded or held until Enter was pressed — never reaching the remote shell.

Fix

• Call `exitAlternateScreenBuffer()` first in `releaseTerminal()` so SSH runs in the normal screen buffer. Ink's renders (targeting the alt buffer) no longer interfere with SSH's terminal I/O.
• Remove `setRawMode(false)`: the terminal stays in raw mode throughout the transition, so keystrokes are immediately available in the kernel buffer. SSH sets its own terminal mode via `tcsetattr(TCSADRAIN)` which preserves buffered input.

Test plan

• Open an SSH session to a devbox via the TUI
• Immediately start typing a command as soon as the shell prompt appears — characters should no longer be dropped
• Verify SSH session functions normally (Ctrl+C, Ctrl+D, exit all work)
• Verify the TUI renders correctly after the SSH session ends

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