Fix interactive SSH sessions

[joehoyle]

Summary

• resume stdin and enable raw terminal input once an SSM session connects
• ping the active SSM session directly instead of reading this.awsSSMSession
• clean up terminal listeners, timers, and raw mode when a session disconnects
• add focused regression tests for input forwarding, the ~. escape, and keepalive pings
• run tests in GitHub Actions on Node.js 20, 22, and 24

Why

After the CLI's ESM conversion, the keepalive callback attempted to read this.awsSSMSession while this was undefined. This crashed connected SSH sessions when the 30-second timer fired.

Inquirer can also leave stdin paused after prompting, so attaching a data listener alone did not reliably make an established session interactive.

User impact

Users can type immediately after connecting, sessions no longer crash on the first keepalive interval, and terminal state is restored during cleanup.

Validation

• node --test test/ssh.test.js — 3 tests passing locally
• GitHub Actions — Node.js 20, 22, and 24 passing
• workflow YAML parse check
• git diff --check