FEATURE: h8 kubectl exec --script for multi-line commands

## Problem

Multi-line shell commands through `exec` are nearly impossible due to quote escaping hell. During the Grist migration, every attempt to run more than one command failed silently:

```bash
# Broken — quotes inside quotes, \$ escaping, shebangs
h8 kubectl exec pod -- sh -c '
mv /persist/home.sqlite3 /persist/home.sqlite3.bak
tar xzf /persist/backup.tar.gz -C /persist
chown grist:grist /persist/*
'
```

Ended up having to copy a script file via `kubectl cp` first, then exec it, then rm it — 3 commands where 1 should work.

## Proposal

```bash
h8 app exec grist --script '
mv /persist/home.sqlite3 /persist/home.sqlite3.bak
tar xzf /persist/backup.tar.gz -C /persist
chown grist:grist /persist/*
'
```

Implementation: write the script to a temp file, `kubectl cp` it to the pod, `kubectl exec pod -- sh /tmp/h8-script`, then `kubectl exec pod -- rm /tmp/h8-script`. All transparent to the user.

Stderr from the script should be forwarded, and exit code from the script should be the exit code of the h8 command.