Post-hoc port publish on msb via ssh serve + ssh -L (gap K) #238

Post-hoc port publishing on msb — expose a port on an already-running sandbox without a re-create — via msb ssh serve + OpenSSH -L local forwarding, wired behind acq ports <sandbox> --publish H:G. This closes the last port-capability gap vs. sbx (sbx ports … --publish) and lets acq ports do something on msb instead of printing "re-create the sandbox." Design: ADR-0015.

Why

msb --tree (msb 0.6.7) confirms msb has no post-hoc NAT publish (only create/run-time -p), but does have msb ssh serve <sandbox> — a host-side SSH listener against a running sandbox — over which OpenSSH -L/-D forwarding tunnels a guest port to the host with no restart. ADR-0014 deliberately left this out of scope (create-time -p only); this is the incremental follow-on.

Scope

• Wire acq_backend_ports (msb) to: authorize an acq-managed key once (msb ssh authorize), start msb ssh serve --host 127.0.0.1 --port <ephemeral>, open a backgrounded ssh -L 127.0.0.1:H:127.0.0.1:G.
• Flip ACQ_BACKEND_SUPPORTS_PORT_FORWARD=1 on msb and correct its rationale comment.
• Track and tear down the serve/ssh process pair on acq rm/stop.
• Validate H/G (1..65535, integer) before argv (reuse ADR-0014's validation).

Caveats (from the tree)

• Forwarded connections originate inside the guest, so 127.0.0.1 in the -L destination is the sandbox's loopback, not the host's.
• Sandbox network policy still applies; reverse (-R) and stream-local forwarding are unsupported.
• Listener binds loopback by default; binding beyond 127.0.0.1 is opt-in and out of scope here.
• Uses a dedicated acq-managed key (not the user's personal key).

Verification

Live end-to-end (acq ports against a running msb sandbox, then curl the host port) requires a KVM host and runs via scripts/verify-backends, per ADR-0011 (no nested sandboxes).

Related

• Part of Epic: sbx↔msb backend parity #234 (sbx↔msb backend parity epic — gap K)
• Builds on acq: implement neutral port-publish + background kit vocabulary (ADR-0013) and msb consumer #224 / ADR-0014 (create-time publishedPorts)

AI-assisted (OpenCode); requires human review.