feat(acq): add a neutral interactive-shell verb — acq shell NAME

[basiliskus]

Fixes #383.

acq run NAME always relaunches the recorded agent and acq exec is non-interactive, so an interactive human shell was the one daily-driver moment that still required the raw backend CLI (sbx exec -it <name> bash). This adds the neutral verb:

• sbx: acq shell NAME → sbx exec -it NAME bash (sbx allocates the PTY).
• msb: routes through a new _acq_msb_shell_exec helper extracted from the two identical shell launches _acq_msb_attach already carried — so the verb gets the agent user, PTY, workspace cwd, sane $SHELL, and SSH_AUTH_SOCK injection (when the host agent is forwarded) for free, and the attach path loses its duplication. A raw msb exec shell gets none of that.

Scope note: deliberately minimal per the issue — no attach-time heals on shell entry in v1 (acq run remains the heal point), and no -it on acq exec. Per-verb --help (per the repo's per-verb help convention) and the three how-to docs are updated; the sbx how-to's "stays raw" note now lists only sbx rm --all.

Verified: 4 new dispatch tests (red first), full offline suite green (354/354 serially), and a live E2E — a scripted PTY drove acq shell into a real sbx v0.39.0 sandbox and got SHELL-E2E-42 from agent computed guest-side.

AI-assisted (Claude Code [claude-fable-5]).

[mogul]

Thanks for adding the neutral acq shell NAME surface. The implementation shape looks sound from static review, but I found a couple of issues that should be fixed before merge.

Findings:

1. Low: durable code/test text uses bare tracker references. New comments and test names include bare #383 references, for example acq.backends/msb.sh:3696, acq.backends/msb.sh:3714, acq.backends/sbx.sh:495, and test/bats/30-dispatch-routing.bats:30, :35, :40, :47. Repo AGENTS.md requires durable code comments/docs to avoid bare issue/PR references and instead use self-contained prose or ADR references. Please remove/rephrase those bare tracker references.

2. Low: docs/howto/msb.md:191 still documents the old/non-neutral interactive-shell path: acq run <name> and raw msb exec -it <name> -- bash. That conflicts with the new acq shell <name> verb and with the implementation, which uses an agent-user login shell with PTY, workspace cwd, sane $SHELL, and optional SSH_AUTH_SOCK. Please update the table to match the new neutral verb and the actual raw equivalent.

Verification run during review:

• git diff origin/main...HEAD --check passed.
• bash -n acq acq.backends/sbx.sh acq.backends/msb.sh passed.
• shellcheck --severity=warning acq acq.backends/sbx.sh acq.backends/msb.sh passed.
• ./scripts/test-acq-bats test/bats/30-dispatch-routing.bats passed: 29/29.
• ./scripts/test-acq-bats passed: 354/354.

Earlier note: Bats initially could not run because vendored submodules were not initialized in this worktree. After initializing test/vendor/bats-core, test/vendor/bats-support, and test/vendor/bats-assert, the focused and full offline suites passed.

[basiliskus]

Both fixed in 04b1a07: bare tracker references stripped from the new comments and test names per AGENTS.md "Durable References", and the msb Common Commands table's interactive-shell row now shows acq shell <name> (the sbx table and the attach example were updated in the feature commit; that row was missed). While re-reading AGENTS.md I also brought the branch into compliance with the sections adjacent to the one you cited: commit messages now use fully-qualified issue references (reworded, hence the force-push), and the PR description carries the recommended AI-assisted disclosure. The branch is also rebased onto current main. Dispatch file 29/29, shellcheck clean.