acq shell/attach on msb launch /bin/sh -l, so the user's bash environment (.bashrc, rc.d drop-ins, prompt init) never loads #426

Summary

On the msb backend, acq shell and the interactive attach path exec /bin/sh -l with SHELL=/bin/sh (_acq_msb_shell_exec, and the parallel attach exec). On Debian-family images /bin/sh is dash, which never reads .bashrc — so everything kits wire into bash (rc.d drop-in loops, prompt init like starship, nix PATH additions in .bashrc) is silently inert, and the user lands in a bare $ prompt. The same sandbox contents through the sbx backend get an interactive bash and the full environment.

Repro

1. Apply a kit that wires ~/.bashrc (e.g. an rc.d drop-in loop with a prompt initializer) and create an msb sandbox from a Debian-family image.
2. acq --backend msb shell <name> → bare dash $ prompt; none of the .bashrc wiring ran.
3. Typing bash inside the session brings the full environment up — proving the guest-side wiring is fine and only the launched shell is wrong.

A second wrinkle: bash -l doesn't recover it

acq's agent-user setup appends export SHELL=/bin/sh to ~/.profile as its only content. A login bash therefore reads that .profile and stops — there is no Debian-style .profile → .bashrc bridge — so even a user who knows to start bash gets the bare environment unless they start it non-login.

Proposed fix

Follow the passwd-driven convention that sshd, machinectl shell, toolbox, and distrobox use: the environment declares its shell in the account database, and the transport honors it.

1. When _acq_msb_ensure_agent_user creates the agent user, set its passwd shell to bash when the image has it, keeping /bin/sh otherwise — the portability fallback moves to user creation, where it's decided once.
2. Have the shell/attach paths exec the agent user's passwd shell with login semantics (and set SHELL to match), instead of hardcoding /bin/sh per session.
3. Write a Debian-skel-style ~/.profile — one that sources .bashrc when running under bash, as /etc/skel does — instead of the current bare export SHELL= line, so login shells behave on images whose baked home ships no .profile.

A per-session probe-and-prefer-bash (bash || sh) would also fix the symptom, but it's the weaker form: the passwd-driven design keeps the call sites trivial and matches what users expect from every other shell transport.

Related

Third member of the msb exec/attach session-parity family, with #421 (working directory) and #425 (TERM/COLORTERM): each is session context sbx provides that the msb path drops, and all three fixes land in the same few functions — one small parity PR could close the set.