refactor(mutagen): one mechanism — pair on ssh, nothing on a schedule

[gapul]

~/Sync/<host> is supposed to mean "a machine I have actually talked to". The launchd agent broke that for the one host in peers: it ran at login and hourly, and mkdir -p'd the directory whether or not that machine had been touched — even on the many attempts where it then failed to create a session at all.

Since #179 the ssh path already covers pairing, so the agent, the peers list and the per-peer script all go. 89 lines deleted, 27 added.

The one thing peers carried that the ssh path did not is the -sync alias. That is now looked up rather than declared: if ssh_config defines <host>-sync, that entry is dialled, otherwise the host itself is. The convention is documented in the header instead of being encoded in a list of one.

Behaviour after this:

• a directory under ~/Sync appears the first time you ssh to that host, and never otherwise
• a failed pairing leaves nothing behind (the empty directory is removed again)
• existing sessions are untouched — they live in the mutagen daemon, not in this module

Checked against stub ssh / mutagen with a fixture ssh_config that has both mvrx-nolang-dev and mvrx-nolang-dev-sync:

ssh mvrx-nolang-dev        -> create mvrx-nolang-dev  ->  mvrx-nolang-dev-sync:Sync/MacBook-Mini
ssh rpi4                   -> create rpi4             ->  rpi4:Sync/MacBook-Mini
ssh 10.0.0.1               -> (nothing)
ssh mvrx-nolang-dev-sync   -> (nothing)

Note the local directory keeps the host's name in both cases; only the dial target changes.