fix(mutagen): dial a sync-only SSH alias so the session can actually be created

[gapul]

mutagen sync list is empty and the log has 14 consecutive SKIP: could not create session entries going back to the day the module landed. The sync has never once worked.

Neither cause is in mutagen:

1. No agent. The private key isn't on disk (it lives in the Bitwarden agent), and launchd does not hand SSH_AUTH_SOCK down to its agents. From launchd, ssh has no identity at all and falls through to a password prompt that nothing can answer — hence unable to prompt: prompter not found in the log.

2. Forward collision. Host mvrx-nolang-dev carries 26 LocalForwards plus ExitOnForwardFailure yes. Whenever a normal dev session already holds those ports, every further connection to that host exits 255. Reproduced by hand:

   $ ssh mvrx-nolang-dev 'echo hello'
   bind [127.0.0.1]:18100: Address already in use
   Could not request local forwarding.
   $ echo $?
   255
   

   So even with a working agent, the sync would break exactly when the machine is in use.

Fix

A sync-only alias in the sops-managed ssh_config: mvrx-nolang-dev-sync, same host, no forwards, and IdentityAgent pinned so it does not depend on the caller's environment. mutagen-sync.nix gains a sshHost field and dials that; the session name and ~/Sync/<host> directory are unchanged.

Verified before committing, with the forwarding session still up:

$ ssh -F <new config> mvrx-nolang-dev-sync 'echo REMOTE_OK; hostname'
REMOTE_OK
sean