Enable Ctrl+Z (job control) in Podman interactive mode

[phryneas]

--- Human note:

I wanted to be able to drop out of interactive sessions into the container, to run commands, see what's available in the container for Claude to use and essentially have a good starting point to debug the whole thing.
This isn't really beautiful, but works nicely for me. Still, please test it by going through the test plan before merging :)

--- AI PR:

Summary

• Wraps the agent command in an interactive bash session with set -m (job control enabled), so Ctrl+Z suspends only the agent process while bash retains terminal control
• The init script is written to /tmp/.sc_init.sh via a base64 round-trip to survive any special characters in the command args
• PROMPT_COMMAND resets the terminal with stty sane on resume, and exits bash once the agent job is gone (normal exit) — but not when it's merely stopped, so fg correctly resumes it

Test plan

• Run sandcastle in Podman interactive mode and press Ctrl+Z — agent should suspend and drop to a shell prompt
• Type fg to resume the agent — it should continue from where it left off
• Verify normal exit (no Ctrl+Z) still works: bash exits cleanly when the agent finishes
• Verify non-TTY mode (piped stdin) still works: init script is still written, bash runs non-interactively

🤖 Generated with Claude Code

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[phryneas]

Oh, I just wanted to add: feel free to close if it doesn't fit the direction you're going :)
I have a local pnpm patch for this going on, just wanted to contribute it back in case you think it might be useful.