kitty annex: thin-client (zenbook) legs for Ctrl+B / Ctrl+G + roaming

[mecattaf]

Follow-up to #49 / #50 (both shipped + closed via #71, verified on the coordinator for Ctrl+B and Ctrl+G on both claude and pi). These are the deferred thin-client (zenbook/worker) gaps, documented precisely in #71 and split out so the lineage stays traceable.

• Ctrl+G on thin-client — kitten ssh does not forward remote control for the abstract socket listen_on unix:@kitty-{kitty_pid}, so kitten @ launch from inside claude/pi on the zenbook can't reach the socket. Fails safe (nonzero → prompt kept) but the composer won't open. Fix: a ~/.config/kitty/ssh.conf enabling forwarding + a non-abstract socket path, or an equivalent.
• Ctrl+B file-open on thin-client — the selected-path signal (pick file + ANNEX_PICK env) doesn't cross the ssh boundary (written on coordinator, watcher reads locally), so selection can't spawn the viewer window remotely. Intended fix: a terminal-native channel — the annex nvim emits a kitty SetUserVar OSC that rides its stdout through zmx to the local kitty window (same trick as the pass-through marker).
• Ctrl+B annex cwd on thin-client — annex nvim may open at the coordinator's SSH-login $HOME rather than the project root (cwd not forwarded over the ssh leg of annex-cmd exec).
• Ctrl+G roaming — KITTY_LISTEN_ON is frozen into the zmx session at create time; re-attaching in a different kitty window targets the original socket (fails safe if gone). Belongs with the workspace-roaming work-package.

Also worth deciding here: the #51 tension — the opened-file window currently uses the server-side zmx spawn path (per the 2026-07-14 handoff), which contradicts REMOTE_NVIM_DECISION's local-nvim+rsync for durable thin-client editing.

Verified-working baseline (coordinator) is #71.