fix(device-utils): stabilize remote shell sessions (#32)

* fix(device-utils): stabilize remote shell sessions

Improve EX/SRX remote shell handling by waiting for the initial shell
output before sending the first keystrokes, buffering that output for the
caller, and ignoring websocket close races during send.

Add ShellSession.send_commands() as a convenience wrapper for sending
multiple commands without manually appending line endings.

Replace the sshkeyboard-based interactive shell with platform-specific
terminal input loops, including POSIX raw-mode handling and Windows console
key mapping. Remove the now-unused sshkeyboard dependency and update the
lockfile.

Update shell tests and README documentation for the new behavior.

* bump version to 0.63.2

* fix: remove unnecessary import in shell session example

* fix(docs): clarify interactive shell behavior and requirements

* test: add unit test for posix input loop in interactive shell