Harden SSH command execution and temporary files

[santiagocasas]

What this changes

This tightens up the places where the plugin builds SSH commands and temporary files.

The old code passed interpolated strings to a local shell. That meant a specially crafted
host or command could be interpreted locally instead of being handed to SSH as plain data.
The files used to disable local tools also had predictable names under /tmp.

This PR:

• uses execFileSync() with argument arrays for SSH connection management;
• validates SSH host aliases before using them;
• passes the whole remote Bash command to SSH as one safely quoted argument;
• creates a private temporary directory for disabled-tool files;
• uses random names for blocked write targets; and
• creates the sentinel file with 0600 permissions.

Commands containing pipes, redirects, &&, or similar shell syntax still work on the
remote host. They are no longer evaluated by the local shell while the SSH command is
being assembled.

I kept this PR limited to the command-execution and temporary-file hardening. Connection
health checks, fail-closed behavior, and the launcher are separate follow-up PRs.

Checks

• Ran git diff --cached --check.
• Reviewed the generated SSH argument lists and command quoting.
• This repository does not currently have an automated test suite.

AI assistance

The code and this PR description were generated with help from GPT-5.6-Sol.