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

[erfanium]

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:

# 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

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.